Something that randomly ocurred to me is that I really freaking hope there's a way to speed up the process of unlocking character trophies. Getting 2 trophies for every character by simply going through each of the main single player modes, once per trophy becomes a hell of a grind ridiculously quickly. And also I feel like it kinda discourages you from replaying the mode as your favorite character, because you wouldn't be unlocking another trophy for them. I got all Classic Mode trophies on the 3DS, but I couldn't bring myself to do the same thing once again for All-Star Mode, and certainly not to repeat it on Wii U, where I still don't have all character trophies even after all this time.
With 68 character (or 65 considering Pokémon Trainer is treated as one) and two "main" single player modes, you're gonna have to do a single player run 130 times. And probably more considering this is very unlikely to be the full roster. That's completely freaking ridiculous if you ask me, not to mention that it could potentially be even more if they decide to include a third "main" single player mode with unique trophies, like Melee did with its Adventure Mode.
I think to potential solutions are 1: Have Classic and All-Star give the same character trophy, so you don't have to waste so much time per-character, or 2: Allow you to choose multiple characters on each playthrough to unlock multiple trophies at once, with each character being your stock.
Option 2 is my favorite. They could even play with the idea of a risk-vs-reward system that lets you use more characters on higher difficulties, but only letting you unlock trophies for the characters that "survive". Like, easy mode, select two characters. You're not going to lose because it's easy mode, so you'll get two trophies. Normal mode, select three characters. It's a bit harder, so you might lose a character or two, you'll end up with potentially 3 trophies, but if you screw up you'll only end with just a single trophy. Hard mode, select four characters. You're very likely to lose a few fighters, but if you're good enough at the game you can potentially make the process twice as fast than if you were playing on easy, as yout get twice the trophies if you lose no lives. But since it's hard mode, if you're not good enough at the game, you're very likely to win with only a single trophy. The only weird thing about this idea is that you get more lives on harder difficulties, which sounds kinda backwards, so I guess the change in difficulty would have to be pretty extreme for this to work.