Though I do think a Generation 8 'mon is likely, I would rather have a pokemon from a different generation (besides Kanto) instead.
There's Scizor, which is one of my most wanted-s, Grovyle, which could represent the Mystery Dungeon spin-offs, Zoroark, which is/was a highly popular pokemon (at least when I was still big into the franchise), Decidueye, which I really wanted over Incineroar, and even ones like, say, Deoxys, which could, in my opinion, probably play around with a "stance/form change" mechanic more interestingly than Urshifu.
Heck, though Kanto is already over-represented in Smash, I would probably enjoy Kantonian Meowth's inclusion a decent amount, due to the species' significance within the franchise's media.
I dunno, I just don't really like most of the gen 8 designs, to be honest.
Except Snom.
Snom's cute.