Had recently been thinking of this, and I must see if this is shared. 

Ignoring the Trials as those are heavily debatable, a lot of Sun and Moon and Ultra Sun and Moon's content has a lot of what I've seen people ask for going forward. For a few examples

• Exploration: A lot of the more recent Pokemon games have been accused of being too linear. Pokemon SM may not have branching story paths, but there are a number of optional areas that aren't required to go into for the main game, such as 10 Carot Hill, Seaward Cave (SM only), Kala'e Bay, and Haina Desert. These areas contain not only Pokemon that you can't find in the game elsewhere or until much later, but they also hide Z-Crystals in them. 
• Difficulty: Many of the Totem Pokemon were difficult fights, and there are human bosses that are trickier than opponents in the Gens 5 and 6. 
• Side Quests: Weaker than the above two, but there were a lot more little dotted tasks one could do in game for extra money or items.

I could probably think up a few more, and would love to hear from you guys about such things, but Exploration is honestly what caught my eye the most. Especially if one considers the first hardware games to be the Lets Go games, I have hopes that 2019 is more the equiv of Platinum or SM than DP and XY. 

Yes as with every evolution of what a core Pokemon game is since the original titles. Post game is still the only major thing they've still to consistently get right. But's pointless to base the previous gen to predict what the next gen will be like (especially one moving platforms). Apart from Gold and Silver which was a direct continuation/sequel to Red and Blue. Each gen is mostly it's own concept of the standard formulae with a couple nods and some returning characters to battle here and there.

I'm happy with anything as long as is not bland af like X and Y.

Tho Sun and Moon are my second favorite titles after Black and White, if there is anything I can ask from the next gen of Pokemon, is that the main game only gets the new Pokemon and leave post game areas for older gen Pokemon like B/W did, plus the smaller details like pinch music and final Pokemon for gym battles theme.

I hope not. I disliked the tropical setting, HATED the trials, how easy the game was, the constant hand holding, annoying characters, and those Z-moves, oh God, I cringe every time.

Make it more fast-pace if they have cut-scenes, and bring back gyms, otherwise sun and moon are fine.

Absolutely fucking not. SM went the entirely wrong direction, trying to be a Yokai Watch ripoff rather than sticking to Pokemon's core. The next game should drop the hand-holding and return to a more open feel as the first five generations had. Drop the "super happy nice friend time!" rival every game 3DS game had too, it's beyond boring at this stage; along with that, drop the Z-moves, they add nothing to the game. Include difficulty options (A basic feature most games have had for the past couple of decades), include all Pokemon, include post-game content (do not leave it out because 'people have shorter attention spans').

Not really, the bad things outweigh the good imo (compared to previous Pokémon games).

+ Exploration (although I didn't actually notice much difference in linearity between Sun & Moon and X & Y): I definitely don't want an open world game, but some more side-areas to explore would be nice.
+ Difficulty (but again, there was barely any difference between S&M and previous games): Pokémon doesn't need to be hard, but recent games have become too easy.

- Trials: they certainly were an interesting idea, but ended up being not nearly as exciting as the gyms.
- Z-moves: I didn't like Mega Evolutions, but I like Z-moves even less.
- Cutscenes: there were way too many in S&M, and they usually were very slow and boring. They obviously tried to spend more time on the story in this game, but the actual plot was equally weak as in other Pokémon games. I hate long and boring dialogues, I prefer games that can tell their story through their world.
- Graphics: I didn't like the change to 3D that X&Y made, but at least it still had a certain charm to it. Sun & Moon looked horrible, and the Let's Go games are even worse.
- Region: this is my main issue with Sun & Moon. The region design is very important in Pokémon, it's what distinguishes the games from eachother. While Kalos was probably the most charming and interesting region in the whole franchise, Alola was the most boring one since Kanto. The main reason for this is their attempt to make the scale more realistic. Because they didn't make the region much bigger, there's a lot less stuff to see and visit than in previous games. For instance, the number of towns and cities is the lowest since the first two regions: Hoenn had 16, Sinnoh 15, Unova (BW2) 19, Kalos 16, but Alola had only 9. The number of inhabitants per town is also lower than in other regions.

TL;DR: I want more X&Y stuff and less S&M.

Pokemon Sun/Moon is the second best generation, only behind gen 4 imo.

I loved a lot about the games.
+more optional areas (like the desert and the inside of the mountain)
+Trials. The totem 'mons felt like actual boss fights, more so than gyms.
+region. It felt more lifelike and oozes with personality. Not the boring snoozefest X/Y was. There were less cities, but each of them feel unique from each other rather than just changing the color of the house's roofs and call it day.
+art style. Getting rid of the stupid chibi style and having a look that closer resembles the anime
+getting rid of HM's, finally
+Pokemon designs are much better, as opposed to the shitfests we got in B/W and X/Y


-Festival Plaza. A huge step back from PSS
- Too many unskippable cutscenes

Overall though, these games brought the series back up after the tragedy that was X/Y.

This statement doesn't make any sense when Lets Go is using the same Chibi style from X/Y...

Lets Go is borrowing way more from X/Y..

i agree with you on second best being gen 7 , but you mean behind gen 5 though right?

:p

(also get PSS back,best thing about X/Y)