Looking at past Smash games with third party, Brawl had 2 new companies (Sega and Konami), Smash 4 had 3 new (Capcom, Namco and Square) and Ultimate had 3 new (Microsoft, SNK and Disney). I wouldn't say this is a pattern that can't be broken, but I think having 2 or 3 new is fair considering that the more you add, the more licencing becomes hard. So I would say that the more likely to join would be Level-5, Koei Tecmo and either Falcolm or a indie rep (like Yacht club games, Brace yourself, Supergiant, Toby Fox, Team cherry to name a few).
That being said, tbh I could see having actually less new companies in the next game, because what we have right now have a huge library of characters to pull from. Esepcially Capcom, Sega and Microsoft it feels like we've barely scratch the surface of what can be add for them.