lol that title
If I was Director I'd go Street Fighter 3 on its ass. Don't get me wrong, I adore most of the current roster and I think it's great they managed to get almost every previous fighter in Smash 4, but the same fighters again with 5/10 new ones and prettier graphics isn't enough of a shake up. Yeah, I do realize that with Smash being a Nintendo all-star kind of deal makes it hard to swap most of them.
But I guess with just 25 returning you'd end up cutting a whole lot anyway. So what I'd do have just have the main character or mascot of each already presented franchise return:
Mario, Donkey, Link, Samus, Yoshi, Kirby, Fox, Pikachu, Captain Falcon, Ness, Marth, G&W, Pit, Wario, R.O.B., Olimar, Villager, Wii Fit Trainer, Little Mac, Duck Hunt. Which would already make 20 characters.
(e: wait, how could I forget the IC?! well they'll be secret character #31 then).
Mega Man, Pac-Man, Sonic, Cloud and Ryu as returning guest characters would make a nice veteran 25.
For the remaining 5 I'd either go all Nintendo (Inkling, Takamaru, Jill Dozer, Isaac, Ray MkIII) or just add a few other NES era guest characters, such like those nicely listed in the NES Classic Mini (Bubble Bobble, Ghost 'n Goblins, Castlevania, Ninja Gaiden, Double Dragon). Flip of the coin!