To be honest, I honestly wouldn't be that upset if Mewtwo was the one and only DLC character. I'm already hugely satisfied with Smash 4's character roster as it is, and Mewtwo was basically the last fan-demanded character that I felt that actually did need to be in the game.
My only complaints with the roster is mainly just some of the cuts. Squirtle and Ivysaur were unique characters, but with Pokemon Trainer and transformations in general gone, there's not much of a point to them. Ice Climbers deserve to return, but they're probably going to have to sit it out until Smash 5, and hopefully by then Nintendo has competent enough hardware for their handheld systems to run them. I'd like Snake back, but I don't know if that's possible, or what MGS's future even is, thanks to what's happening with Kojima and Konami.
Wolf and Lucas at least seem possible, considering they're generally easy to make. One concern I have is that they just might not be considered highly exciting or popular DLC choices, and while I most certainly want them back (Wolf was my main in Brawl), I can kinda see why. I mean, they're both semi-clones, and one of them is a protagonist of an obscure japan only game and the other is a tertiary Star Fox character. I'd welcome them both back with open arms, but I fear Sakurai and the other Smash devs might not seem them as compelling choices for DLC.
As far as newcomers are concerned....I guess I'd like to see a Rhythm Heaven character of sorts, but again, would that be a fitting DLC character? I agree Splatoon should be represented in Smash, but that's another thing I believe we should wait on until Smash 5. Olimar wasn't shoved into Melee as a playable character just because Pikmin was fresh and hot at the time.