While I got my dream character and could be content with it ending here, I feel like the potential for Ultimate is to continue as long as possible. As Sakurai said, if they make another game, there will likely be cuts. Which is why after the E3 2018 reveal of "everyone is here" made me hope they would use this as a base to continue as much as possible, before balancing becomes impossible. Why make a new one when you have everything?
That being said, after DLC started, I saw perhaps why--a new game gets better attention, and so it's more profitable to make a new one, even though you have the perfect setup to continue Ultimate as much as possible. I hope DLC sells well enough to get a season 2--Sakurai's statements make me think we were lucky getting a Fighter's Pass at all (thank you, thank you, thank you), so I'm doubtful, but hopeful.
(also, with Banjo & Kazooie, I'm set, and genuinely cannot ask for anything ever again--I feel this way simply for other's who would benefit from content beyond the Fighter's Pass)