Let me try and use characters who weren't multi-platform, even though people like BD and Heihachi are multi.
Ellen/Keats from Folklore - Folklore doesn't get near enough credit as it should. Furthermore, rather than one or the other, I'd rather have both, even if one just comes in as a NPC ala Kuma, or a super themself like Clank. Using the Folklores would make for some awesome supers.
Max from Dark Cloud 2 - DC2 was just as good as 1, and Max provides a better shoe in for than Toan due to the fact that he actually had some dialogue. Plus, DC 2 had enough unique features to make for such a great moveset.
Pecoros from Breath of Fire 3 - Peco would be my all time wish for getting in, desite the fact it would never happen. He has breath attacks, transformations, and he was just such an endearing character that I think we'd all have a nosralgia tear or two from most of us.
Ramza Beoulve (sp?) from Final Fantasy Tactics - Ramza is without a doubt the only sword user I'd be on board for adding, if only because FF does deserve a rep in this game, but Ramza has enough of an arsenal with his job changing (Can you imagine going from squire to monk to time mage, like Toro but good?) Level 3 could involve Squire Ramza casting Ultima.
Ico - the only rep here from a game I have admittedly never played, but I imagine He'd be more likely than any of my picks.
I think it would be cool if the game could just get yearly character updates. 30 new characters and new maps and such for 60 bucks. And the previous years drop down to 20 or something. That way in 3 years or so you would have a completely epic and ridiculous cast of characters.