Outside the Pokémon series, I've known for a while the exact characters I really want in Ultimate, but when I look at which Pokémon I'd want, I can't seem to decide.
I remember, a few days ago, answering a fan poll about newcomers in Ultimate and scrolling through the "characters you want the most" section, and when I saw Gardevoir, I stopped. I asked myself, "Is Gardevoir really this popular to get into a poll?" The more I thought about it, the more I wanted to give a vote to Gardevoir, but after some hard consideration, I used it for another Pokémon that I think would be more likely to make the cut, probably Decidueye.
There isn't really any specific Pokémon I want to become a fighter. What I can say, though, is that my 3 favorite Pokémon generations are 3, 5, and 7 (with 3 being my #1 favorite), and those 3 are the only generations to not be represented in Smash with fighters. I know Pichu's sort of a joke character, but he/she's still from Gen 2. I also know that I love Grass-type starters, which I feel got shafted a bit compared to Fire-types across the series, and especially compared to Water-types.
What I want, then, is really any Pokémon that's from Gen 3, 5, or 7, and/or a Grass-type starter. Obviously Gardevoir fits this, and while Gardevoir isn't necessarily one of my favorite Pokémon, I definitely like her a lot, and I think she's one of the best Gen 3 Pokémon/Fairy-types.
I don't know if I'd commit to putting my full support behind any single Pokémon, but I'd be pretty happy if Gardevoir was confirmed playable for Ultimate.