I'm not a fan of posting negativity about people's wants for Smash, but I think the game is saturated with third parties. The Smash 4 roster is 10% third party.
Personally, I don't want any more third parties in the game. It was supposed to be Nintendo's All Stars, and there's still a LOT of Nintendo series to explore in Smash, plus ones that haven't been fully explored (Zelda, DK, Metroid, Fire Emblem [no archer/axes/lances/etc.], Pokemon [no grass type], etc, etc.)
My preference for third parties, is that they not exist.
I loved Smash 64 and Melee, they got on just fine without third parties.
Although I was heavy into Sonic during the Game Cube era, I kinda did want Sonic in melee, but I digress.
I didn't really like Brawl... they scrapped Mewtwo which was one of my favs... they did bring on Sonic.. which was cool but then I felt Snake didn't fit the tone at the time, so I had mixed feelings on third parties.
By the time Smash Wii U rolled out, I had accepted the concept of third parties.
I really enjoyed how it only included Japanese companies.
I hope Switch continues that trend, there's a few Western based companies that have characters I like, but there's also a mass of western owned properties I would just roll my eyes at.
For some reason I really enjoy specifications and confines for third parties. I don't know why that is, but I do.
Any way, If third parties return, which I'm fairly confident they will... I hope Sakurai continues the Japanese trend... and hopefully appeases my OCD by giving each of the companies involved 2 reps. I wouldn't mind seeing Koei get in this time... or even the smaller guys like Natsume and Level 5.