I know that everyone says that no matter the character, plenty of people would be mad no matter what, but I fell that goes the other way too. I'm sure no matter who the next DLC character ends up being, they will have a ton of people that welcomes them with open arms. I guess it's just easier to focus on the negative side of things. So many people who didn't know Terry thought he was sick, for example, but it was easier to get annoyed at those who thought he was just some shill character or something. If a Fire Emblem character can still be seen as "Yeah they're kind of neat still" by the majority of people, pretty much any character will have a mostly positive response.
As for who'd be good for E3, I guess that's a little tough to truly decide. Most E3 Smash showings had major franchises or characters like Mega Man, Villager or Hero or Ridley, and granted Palutena got higher billing than Pac "I eat Ass and get Cash" Man. Hell, Brawl had characters like Meta-Knight and Wario, pretty damn popular Nintendo characters, oh and ZSS too, but that's Samus again so I don't know if people want to count that.
Who's labeled a major character? Crash, Master Chief, Lara Croft, Dante, Heihachi, Ryu Hayabusa? I think they all fit into that category, but I'm sure someone could make a case for Phoenix Wright or even 2B.