Whattttt!!! Yeah not as popular, but not anymore? Ninty Land/Miiverse including having mii's playable in NSMBU? Dontget me started on 3DS!

That said, they wouldn't be bland in SSB. They can have a huge arsenal! Look at villager, he could of been in the same situation if i were to discuss about him pre E3!

One thing in SSB is you never underestimate Sakarai. He makes characters from absolutely nothing into something (*if you dontcount the abomination  Ganon lol)

Ex: Star Fox + C. Falcon (both of which had literally no type of moveset)

And those of which seemed impossible (in this case Mii's)

Ex: Villager + WFT + Mr Game and Watch + R.O.B

:)

Also there's the novelty of being able to fight alongside your favorite characters, aswell as being able to beat up people you don't like!

I see a lot of potential in Miis, actually.

1. For the lulz. Remember the Reggie for Smash meme? There you go, Reggie in Smash. I can't wait to see the reactions when downloadable Reggie Miis are made available and Reggie is everywhere online. Other funny things will happen as well, I'm sure, as people get creative with the Miis.

2. Customizable looks. Sort of plays into the first thing, but it'll be fun. People will get really creative. We'll even see some "characters" that wouldn't normally be in a Nintendo game. Online play should be interesting. Oh look, there's Goku, and there's Master Chief, and there's a humanoid Spyro, and there's Sora, and Geno made it in after all. They're all Miis, but still.

3. Customizable moveset. What better character for this than Mii? This could take a lot of forms. I imagine the specials would be a set of Mii moves to choose from, while the standard moves would have a huge variety, borrowing from other characters much like the fighting wire frames did. Ever played a Tekken game? I seem to recall there being a robot character that worked like this in Tekken Tag Tournament 2 called Combot, with customizable moves you can take from other characters. Guess who's developing the new Smash? The Tekken Team. High level play will involve Miis a lot probably, with people finding the best combinations of movesets and making super Miis.

4. Due to 2 and 3, modding will be incredible for this game. Reskin a Mii to look like whatever, give it whatever modded moves you want, and tadah, instant new character.

The way I think it will work in the vanilla game will be similar to choosing a name. Just as you can set the controls in Brawl for a specific name, you can choose Mii, then scroll through a list of Miis, or choose to add a new one. Upon choosing to add a new one, you'll go to a creator screen where you can import a Mii or create one on the spot, then create the moveset. Miis will probably be really cool if they do this.

Of course, they could also choose to just make a generic Mii and give it a generic Mii moveset from the Mii games and call it a day, which would be a horrific waste of a slot.