So say there's five more stages, I'd expect most to 'belong' to a newcomer, and for the final three I would pick Incineroar, Elma, and a third party (Erdrick, Sora, Dante, Banjo, or Geno).
So, Sun/Moon, Xenoblade X and 'third party'.
For the other two stages, I think it's unlikely that they'd use an unrepped series that could be included via a DLC fighter, as it'd be better to sell a DLC pack with fighter + stage, so I don't see something like Rhythm Heaven or ARMS having a stage in the base game.
I think it's more likely we see stages for things like Nintendo Land or Excitebike.