The only 3rd party I could see adding honestly is Rayman due to the whole Ubisoft/Nintendo love affair going on right now.
Yes Pig Ganon would be awesome but I can think of a bunch of first party characters I'd still love to see like
- Dixie Kong, how she hasn't been added this far in is baffling to me? The 2nd leading lady of Nintendo (after Samus of course), Tropical Freeze recently got a Switch release and she is a major part of that game, starred in 2, MILLION selling games on the SNES. Has had made many appearences throughout the years whether in DK or Mario spin-offs. She could easily be a semi-clone of Diddy.
- DJ Octavio, I mean they've been adding villains might as well add the newest Nintendo baddie of one of Nintendo's biggest new IP's at the moment
- Paper Mario, Could have an extremely unique moveset that revolves partners, stickers, his hammer, cards, etc. If we can have 3 Link's we definitely can have 3 Mario's!
- Impa, besides Link, Zelda, and Ganon, Impa is the most reocurring character in the series. While originally I thought she could be a Shiek echo, if they base her moveset/design off of Hyrule Warriors she could be unique or at least a semi-clone.
- Sylux, while as a character he isn't that well known or developed that much and Metroid got 2 new reps this game. I really do feel Nintendo is gonna go the more "promotional" route this time. What better way to promote Metroid Prime 4 than by adding Sylux, the presumably main villain of that game.
- Bandana Dee, people said "Why add a Waddle Dee he's generic" well that lame argument is finally thrown out the window when Piranha Plant was added. Kirby deserves his player 4 to complete the modern Kirby quartet. He is EXTREMELY popular in Japan so I see no reason why he can't be added. They could literally rip his moves/character from Star Allies and he would be fine.
- Captain Toad/Toadette, The fact that Toad hasn't been playable in a Smash game yet is kinda appalling. He's a Nintendo All-Star and deserves to be there. I prefer the Captain moniker as it makes him a bit more distinct. They could either A. Have Toadette as an alt or B. Do an Ice Climbers deal and have both Toad and Toadette working in tandem.
plus a few more I don't feel like typing out.