My wanted Third Parties:
1. Slime (The Most Important, no provisos)
2. Banjo Kazooie (Assuming Nintendo get the rights for it and Banjo Threeie is a thing... or I win big on the lottery and do it for them...)
3. Simon Belmont (Assuming Konami sort out their act and release some Wii U games... at least port Lords of Shadow)
4. Pac Man (Assuming he has a cool move set, and they do the right final smash)
5. Bomberman (No provisos, he can get in even if Konami's a ****, I'd just not be as excited as for Simon)
6. Layton (Assuming there are more Layton games, and he has a cool moveset)
7. Tails (Assuming Knuckles, and the series remains Nintendo Exclusive)
Knuckles (Assuming Tails)
8. Travis Touchdown (The language would have to be toned down mind you)
Geno and Pulseman would be great Assist Trophies mind you, just not fitting of a playable slot in my eyes.
I can't think of any other 3rd parties I'd want, plenty I'd be okay with, but ya.
Ultimately I think about 1/6th of the roster is the limit for third parties. I'm quite generous about it, but they have to be exciting, and fitting... and most third parties don't hit that, so the limit won't be reached. The problem isn't being 'overrun' with third parties, but third parties getting in when there are more deserving first parties. If Layton got in and Ridley didn't then even if there were only 3 third parties I'd think it was too much.
Ultimately there's a huge step between Bomberman and Layton, where a bunch of first party Nintendo characters need to go before they'd be acceptable to me... and Slime is the only third party who justifiably could have more priority than any Nintendo newcomer in my eyes. Given the provisos and the First party importance are adhered to there's, in my opinion, no chance of it stopping feeling like a Nintendo fighter.
As for the Raymans, Snakes and Final Fantasy characters, they can go hang it in the never-zone of PSASBR2.