Saki because his appearance and weapons are pretty cool.
Little Mac because...well, he's a classic!
Lyn would be cool because iaijutsu rocks.
Isaac is alright, I guess, but I'm not a huge fan of Golden Sun (Although I have played through part of the first game)
And the Advance Wars AT should be playable next game =P It'd be Andy, but the character would be a 'pet' character along the lines of Olimar/Pikmin.
EDIT: On the subject of Shadow...I'm sorry, but I'm not even sure we'll see Sonic or Snake back next game...but if they become staples of the Smash series (We can only hope!) any Sonic character BUT Shadow should become a PC. Tails, Robotnik, and Knuckles deserve a PC spot over Shadow any day, assuming they'd even put other characters from third party franchises in