With how big Ultimate is, I foresee Nintendo skipping this series one generation just so that they can reset it. Fans could get mad, sure, but it'll give us a break from this game until we start to miss it, and settle for a reset. I'd say skip Switch 2.0 and restart with the next gen after that.
As far as Waluigi go, I doubt he'll ever be in this game as a fighter. Although, knowing how trollish Nintendo can be/get, I would really like to see a surprise add and show that Master Hand(s) are actually Waluigi's hands.