Banjo's a nostalgiafag pick who missed his chance back in Melee. His reveal in Smash would get a few people excited, but it would pave the way for a Banjo reboot that would actually get people excited.
Simon Belmont is a more plausible and exciting nostalgiafag pick, but nobody's really excited for new Castlevania.
Master Chief hasn't been considered iconic since Reach and nobody really feels nostalgia for him.
Doomguy can't get in without bringing at least his shotgun with him and that's not happening in the base game. Would be extremely hype if handled well.
Shovel Knight's brand has been tarnished by showing up in every indie game crossover. He's a shoe-in for Ouya Rumble. Shantae is Shovel Knight for hipsters and waifufags.
Sans Undertale and Steven Minecraft would be exciting for a large audience (though Minecraft is declining outside of the dedicated legolords), but nobody here wants them.
Sora would be exciting, but Sora without Donald and Goofy wouldn't fit with Kingdom Hearts theme of togetherness, and bringing Donald and Goofy brings us to Goku.
Goku would unironically be the most exciting and only autists don't want him because "muh rules", bringing up Shrek like Anti-Bandana Deefags bring up Fedora Goomba.