Piranha Plant was just confirmed as a playable character. While I love the idea of him and how he looks, I don't think its safe to assume anything at this point. Are more 3rd parties likely? Square is pretty likely, so sure, why not? But for all 5 of them to be 3rd party? I wouldn't take that bet. We've got a lot of characters that can still be playable from existing 1st party franchises.
Another Mario rep isn't impossible (plenty from main series or they could grab someone from Paper Mario)
Another Zelda rep is definitely possible (they could do Impa as a unique or semi-clone rather than a straight Echo)
Another Final Fantasy/Square rep is easy (Sephiroth; Terra or Celes from VI, Crono from Chrono Trigger, any MC or even Jade from Dragon Quest)
Another Pokemon in the form of an Ultra Beast or Necrozma
Another Sonic character (I know people wanted Shadow, but Tails, Metal Sonic, or even Dr. Eggman could easily be brought in to much applause)
Another Fire Emblem character is probably one of the easiest to actually do
The list goes on
Its not impossible, but I sincerely doubt all 5 would be 3rd party characters. To be honest, I'm surprised there wasn't something that catered more solely to Japanese Audiences.