First off, let's be reasonable - characters like Megaman won't appear in Smash Bros. 3 unless Nintendo somehow buys the rights to Capcom, which won't happen. I don't think characters like Sonic, Tails, Knuckles, Shadow, etc. are impossible to put in the game, as I hear (from Kevin) that Sega wants to have their games available for download via the Revolution. That may mean we may see Sega characters in the new Smash Bros. game. Anyway, who would you like to see?
I think Pit (of Kid Icarus fame) will be in it. He's getting a sequel for the Revolution, so it's a good possibility. I'm willing to bet Wario (maybe Waluigi, too) will be playable, as he's become popular in the last few years. Finally, I wanna see me some more Earthbound characters, dang it! Give me Poo!