Going to be honest, I think Smash Ultimate kind of spoiled me with all the returning characters and stages that I'd be reluctant to buy the next instalment. I'm already not convinced that Banjo & Kazooie are going to return so they'll have to impress me with some good single player content and cooler stages to keep me hooked to Smash Bros, otherwise I'll just stick to Ultimate. Even then I'd still be on the fence whether I'll buy it or not because I know that not every single character and stage will return.
An Ultimate deluxe sounds like a good thing but wouldn't they have to get the rights to all the characters again to let them appear in an Ultimate deluxe?
That would depend on if Ultimate DX is considered a new project or not, which is a gray area in itself.
One could make the argument for either - on one hand it would not be a new project because it's fundamentally the same game as vanilla Ultimate (like MK8 DX is fundamentally the same as Wii U MK8), but on the other hand an Ultimate DX would be a a project on another console and thus a "contract extension" of sorts.
Either way, Nintendo's likely to take the safe approach: they would probably want to be sure they can get all the rights before comitting to an Ultimate DX.
So what we're down to Super Mario 3D World and Xenoblade Chronicles X as far s Wii U exclusives worth anything?
Lost World (which was an exclusive) and Rise of Lyric had some potential... but then again, the former had some strange inconsistencies in both story and gameplay, and the latter... yeah. They're reasons if anyone's morbidly curious.
At least Eggman shot a tree in Rise of Lyric and was
insanely proud about that. (Linked commentary / playthrough starts just a little before Eggman shoots the tree, where the commentators question why water-based turrets are being destroyed by water.)