I always wanted Simon Belmont, but I'm kind of afraid he'd be doing too many things similarily to Link to warrant an addition, since I do agree with Sakurai's view that characters should do something unique to be worthy additions to the roster.
Then there's Phoenix Wright, who looked like an interesting enough character to get some kind of crazy moveset like in MvC, even though he'd be yet another third party.
I had never considered it, but agree that Paper Mario could do some interesting things even if all they used were the paper mechanics from his games. Even without partners, he could fold into a plane for his up b, roll up into a tube for shield rolls, use his hammer for jabs and use star power or some sticker for his final smash. He could also have lots of aerial control (but probably not as much as Jiggs, though) to make for a unique playstyle as well.
As much as I love Ridley, he just can't do enough unique things now. He'd probably be some sort of Bowser/Charizard rip-off unless they made him Meta Ridley and gave him lots of hi-tech moves. Even then, he's already a boss character in the game.
Seeing Wolf again would be cool, and they'd save lots of time and money on him since they could reuse his old voice clips, and his final smash could again be a retextured Landmaster with some tweaked variables.
(Italian here) but yeah I'm currently living in South America and I've been living here for a long long time for job issues but not in Mexico.