I feel like Ganon is another good Melee veteran, since he's one of the ones that's had changes, but still feels a lot like himself. Zelda is another example, since she's undergone a lot of changes and is now a good character. The same goes for Bowser.
Falcon would probably be best, though, because a lot more people would recognize him and connect him with Smash. Plus, he appears on the loading screen. And Falcon's had a pretty awesome change made to him, while still being very similar to his Melee self (I'm referring to his taunt after knee, by the way).
A lot of people seem to be excited by Charizard, and splitting the Pokemon trainer up, so that's another thing to think about. Also, you guys need to replace white Charizard with Shiny Charizard. It'd be awesome.
Sonic is a possibility too, because he's pretty cool now. Dedede is also pretty awesome. These characters still retain some of their feel from Brawl and their respective series, which is good.
Obviously what SB said (and Peef mentioning Lucas) is good. I feel like those are the most representative, but thought I'd throw out what some other stuff so there was more to think about.