Hmm, my guess is that we probably won't see another new Pokkén Tournament game during the Nintendo Switch's lifetime. The DX version is still pretty recent, and if the developers wanted to release new content, they could gradually do so in the form of DLC packs. (On that note, I think we're getting another DLC pack this month! ^^)
Any console after the Nintendo Switch could still be fair game, though! I would imagine that a fighting game sounds like an appealing concept to a reasonable number of Pokémon fans, so it's possible that they might consider expanding this title further down the line.