I've owned Splatoon since day 1 when it released. Kinda dissapointed on so many things while I played this. But I'll give Nintendo some slack since it's a new IP. So I expect Splatoon 2 to be WAY better so, Here's my list on How to make Splatoon 2 a WAY better game than Splatoon  already is!

Btw: If you dont want to read ALL of it. ATLEAST read the titles. 

1. Add voice chat: Lol well you all knew this was gonna be on the list. Look... I love Splatoon SO MUCH it's one of my favorite games of all time already. But this is a no brainer, common sense ect ect. Voice for a multiplayer game NEEDS to exist! Especially when you have teammates and friends. Like who cares if people are rude?? It's all over the place and there's no way of hiding it. Nintendo I'd LOVE voicechat on Splatoon. It's too much to create a Skype account and make your friends make skype accounts just to chat with eachother. Like it hurts us gamers that Wii U doesn't even have a Party Chat feature when they existed since last gen. I hope NX will have party chat atleast but it'd be great to have Splatoon to have its own native voice chat. 
2. Make Region games optional: I don't know why we get paired up with Japanese people all the time.  Sure it's popular in Japan but I don't want Nintendo using this for an excuse. "Oh well there's other people from different countries, so no voice chat, sorry". I dont want to hear anything like that from Nintendo. Mario Kart 8 had that option. Look at all the pros! Better connection, same skill levels (Japanese people are VERY skilled and its not fun getting rekted by them), better chances on meeting new people (if only voice chat was open so we can talk to them) and so many others. But lets say if NX comes out and you still have Wii U and there's not enough players in your region to play with  you online.  Just switch to "Worldwide" and there's no problem on finding 7 other people to have a match with!
3. Give us a more dedicated Story Mode: I enjoyed the Story mode alot! I played it over and over until I collected everything. But in my opinion it was way too short. When there's a wifi error or something, no problem! I can play the long Story mode. The story mode was all about obstacles and not really story at all. Take example Super Mario. Super Mario 3D World wasn't focused on Story Mode but was focused on obstacles mainly! Give us beauitful CGI short cutscenes, maybe voice acting and a longer story mode to complete the Story mode experience. Instead of a short story mode with obstacles. 
4. Better mechanics!: The mechanics on Splatoon was decent for a new iP! But I expect more from Splatoon 2! Like lets say you come face to face with someone. and its a 1v1. Maybe adding rolling to the side, dodging the ink to save yourself!  That would be good.  Or ability to jump higher and over your opponent and splat your ink all over their face! That'd be great! A more competitive way of playing Splatoon.
5.  An offline mode of Turf Wars: I don't get how this wasn't on Splatoon. Like why? Instead of the balloon popping and territory taking modes we got. Like what's wrong with battling against CPU's?? I don't get it. But this would be fun!
6. Better Customization for your Squid/Kid: Yeah the whole changing hair and eye colors and gender was cool. But what about hairstyles? face morphing? When playing online everyone looks the same lol. Different clothes and same faces wasn't much of a difference lol. I wouldn't mind more customization for my Squid/Kid.
7. More accurate things:  Like when I mean accurate. I mean no dying beside someone's shooting ink ya know?. When I play Splatoon, I'm shooting in a spot then someone comes near it and dies right beside it. That just shows Splatoon needs more work. Or when I'm using a Special I click the analock stick and my Squid doesn't pull it out like 1.5 seconds later. Or when I'm about to die, I try setting a landmine but I just die... even though I pressed the button like 2 seconds ago. Things like these are FRUSTRATING and would make the game ALOT better. 
8. Remove stage rotations: Like at times I'll feel like playing Splatoon, I'll check the stages and if I dont like those stages I wont play Splatoon. I guess there's nothing wrong with the stages, I dont like playing the same 2 stages for 2 hours and the stages finally change. Look at Mario Kart 8. They let us vote for the stages and its beautiful. I'd LOVE to see Splatoon 2 do this.
9. Add clan matches. Now THIS is another reason to add voicechat. Clan matches. Just get 3 other friends and you and fight the other group of friends!  That'd be fun and voicechat WOULD be needed. Rank up your clan and all the other stuff. It'd be great.
10. Remove Inkopolis News time or make optional: I REALLY dont like Inkopolis News time. It wastes time,  you dont need it and its boring. This is what I seriously do. I start up Splatoon, push back buttons, A,A,A,A,A,A,A,A,A,A,A,A,A,A,A and i tap on "Lobby" on the Wii U gamepad screen THEN I turn on my TV. Like yes I do know its a Japanese inspired thing, but its a video game. If its bad then remove it. I'd LOVE to see "Inkoplis News Time at start on game? Yes? or No?" on Splatoon 2
11. Make Splatfest optional: I don't like Splatfest at all. I always seem to pair up with the low skilled players and I lose most matches with them. The last splatfest I only played 1 match, the Spongebob and Patrick splatfest I skipped because I don't like splatfest. When I hear Splatfest is coming back, I usually play Mario Kart of another game until Spaltfest is over. It's fun with the 2 first Splatfest games but it gets annoying right after that. When you join a lobby you are forced to choose between 2 teams and forced to participate. I know people out there love Splatfest and I know there's people there who hate Splatfest. But I personally call it "Crapfest" lol. No this is not a rant in anyway. But I do personally call it "Crapfest" because I had a bad experience with it.
12. Bring a larger soundtrack: Splatoon had a GREAT soundtrack! Like wow, it was unique, catchy and fun! You heard that boss ending theme? SPINE CHILLING! I expect Splatoon 2 to bring more to the table and break the whole house down with their music!
13. More creative maps: THIS is a no brainer. Like the old maps were cool, but they seemed like war zones. Like imagine having a turf war in a moving CITY... Cars all around, streetlights, other Squids being sprayed on. Turf war in a school building, Park, Piano, Wii U gamepad and stuff like that!
14. Other Nintendo franchise DLC: THIS WOULD BE GREAT! Using Link's crossbow to shoot ink arrows, using Mario's water gun as a Ink gun, riding Kirby's star as a Ink vehicle special. But its not just weapons! ITS MAPS. Nintendo kinda did one already with DK's inspired Splatoon stage. Imagine having a turf war on Rainbow road and hearing a Mario Kart Splatoon collaboration soundtrack. Turf war in Hyrule Castle, turf war in Nintendo Land's theme park. In a Animal Crossing town, Super Maro 3D World stage, Peach's castle, and sooo many Nintendo franchise IPS can get involed in this. All this possiblities can happen! I'd pay good money for these great DLC's.
15. Map creator mode: ... Imagine buying a Full house, with basements, downstairs, upstairs, piano, bed, table, tv, laptop and creating your ENTIRE house and having a 1v1 turf war? or 2v2? Creating your school football field, Or possibly recreating your entire school and having a full 4v4 turf war in your creaction!! It's gonna be like Minecraft! All these creactions will come to life! Spongebob's block, Peach's castle, A Pokemon battle area. What other maps can you create to have a turf war on?? Like wow!
16. Create better gaming lobby:... like why do we wait on a loading screen with names? Ever play Rocket League??  Just splat around a map until the real game is ready. Triforce Heroes had a good waiting lobby. Creative.
17. Have 2 players play online with the same console: Mario Kart had this and man it ever made the game better! It kinda sucks just taking turns when playing online with your friend lol.
18.  Way more game modes: What about a Splatoon without that is focused on skills?? Or capture the flag? Like Rain Maker, Spot taker, Turf war is cool but we gotta have more modes lol. Also give us the Percentage on how many players are playing what. If we see a mode that has 1 percent players then we'll know we'll be waiting a long time to get a match.                        

There's also other stuff I have but I feel these are the most important things that would make Splatoon 2 a masterpiece. What do you think? 
        

Edit: Okay I don't know whats going on with the Numbers. But it goes up to 18.

Releasing it for PC.

Hahaha, this would be best but you know how this business works.

1. Nintendo had their "reasons" to not include it. It was not because it was impossible or the wii u couldnt handle it but because nintendo dont want voice chat. So very unlikely.
2.Sure
3.I dont think story in general is important, but the single player mode should be longer and add more ideas. Splatoon was pretty much the small 3D Mario on Wii u that 3D World wasnt. But it was to short.
4. Yeah they should improve the mechanics from Splatoon for sure. My idea would be to increase speed drastically when swimming in ink on walls, so its a bit similar to wall run mechanics in new cods/titanfall
5. Sure, with bots please.
6. Sure
7. I think thats because p2p. Should add servers for better connections but nintendo really need to improve their online in general. I hope it could still be free then.
8. See my first point.
9. There are team matches
10. Yeah, but again that was added on purpose.
11. Hm idk, splatfest where a great idea but they need to improve the rewards ALOT.
12. Sure.
13. I think the maps are already pretty creative. Hard to improve on that. All are very unique as well.
14. I dont like Nintendo mixing up all their franchises. Should be stand alone.
15. Map creations is always great. See what people achieved on Counter strike (hammer editor) or even on Halo. On consoles the possiblities are still limited though.
16. Yep
17. Yeah, just wast possible on wii u because of the 30fps drop.
18. Yeah agree. Splatoon should include standard modes like Team deathmatch and Free for All. But they shouldnt make to many gamemodes.

DLC incoming. :)
most of them are good, but we can see DLC for those additional game modes. the Splatoon rollout helped as well. let see in Spla2n

wow, ok, not reading all that. here are just a few small details that I would be happy with-

1. improve Squid mechanics. right now we can swim and climb walls. I would like it to be easier to go around corners when moving on walls, and swimming on ceilings would awesome. maybe also make squids a bit more dynamic when moving.

2. Improve on the game's central hub. I would to be able to actually explore Inkopolis and have a bigger immersive world with more stores and feature. Im not saying it has to be open-world, but at least the size of the hub worlds we got in the Mario 3D platformers.

3. Local multiplayer. If the rumors of a standard tablet controller are true, then local co-op needs to revamped. Give us more than the 5 basic maps, no more balloons, let us have local online squads, just everything.

4. Stage choosing. Stage rotation is fine, and honestly it keeps any one map from being over-used or feeling boring. But if we would just choose from the 2 or 3 maps available instead of random ones, that would be nice.

5. The entire upgrade system needs an overhaul. If i get a weapon, I should be able to upgrade it with bonus effects that directly effect that weapon. that way, clothing would be less trouble to customize, and the bonuses would make more sense.

6. Free-play mode. i dont know why but i think it would be cool if you could have a free-play mode where you and your friends could go into any stage you want and just do whatever you want with no time limits. it could help players plan new strategies for specific courses, make it easier to adjust to new maps, and it would be great for when you get super bored and just try to cover the entire map in ink. (we all do it in the test room and Octo Valley, dont lie)

As much as I railed against the lack of voice chat in Splatoon, I didn't really miss it once I played the game.

My recommendations:

Remove rotating maps
Add more maps
Add a few new MP modes
Expand the hub
Add local multiplayer w/ optional bots (I feel like all shooters should offer this)
Ditch Amiibo with extreme prejudice

Those are my big ones.  

I for one actually like the map rotation. Just not the amount of maps per rotation. I think the rotation is a genius idea to keep people playing and keep all the maps fresh. Plus it's almost necessary for Splatoon 1, considering that some players pick weapons based on the rotation (like myself), and you CAN'T CHANGE WEAPONS WITHOUT LEAVING THE LOBBY.

Most of your suggestions I didn't have issue with in the original (I never really have any accuracy issues, as you phrased it, so that might be just an issue of connection or something, and I just use Skype for voice chat, it's cheaper than buying a crappy headset anyway!), but the fact that you can't change your loadout at least in the pregame lobby is disgusting. If I'm in a match with one friend and my whole team is rollers and I happened to like picking rollers for the rotation, I can't be the guy to change to a ranged weapon to balance out, unless I leave the lobby and then force my friend to either stay in or leave as well.

It's ridiculous and decidedly Nintendo level outdated in a game that's very modernized.

I actually think it would be cool if they added a legitimate single player campaign. I understand that's not the point of Splatoon, but I would still enjoy a well-made campaign mode.