Well, usually all of the Pokemon in Smash are usually the most heavily pushed by the Pokemon Company and are usually very popular Pokemon on their own, like Mewtwo and Lucario for the movies that came out awhile ago. And Greninja was voted the most popular Pokemon two or three years ago. And seeing as how Lycanroc is being heavily pushed by the Pokemon Company and is a pretty popluar Gen 7 Pokemon, I think he is more likely than some other options.
But hey, I could be wrong. I was one of the people who though that Ridley was out of the question no ifs ands or buts. And well, needless to say, I was wrong.
So who knows who a Gen 7 Pokemon rep would be.
(inb4 Zerazora gets announced for no reason lol.)
You see, that's where I'm afraid you "might" be right.
Lycanroc is very well pushed by the Pokémon Company, and personally I don't like it very much. More because I don't like Lycanroc in itself as it does not appeal to me (none of the forms), but, the "pushed heavily" part is something I think Lycanroc does have going for him.
Still ... Sakurai is the one to decide, and considering when Sun & Moon came out, it's sort of X/Y scenario again.
We simply have no clue. As said, it could be a similar scenario where Sakurai got some more concept arts and had to decide. Therefore I think likes of Incineroar and Decidueye does hold up a great chance. (Yes, even Primarina at some point)
And the Ultra Beast is the most prominent feature. Therefore, perhaps even Buzzwole.
Tapu Koko does have a prominent role both in the game and in the anime, but I don't think it's that likely.
I'm still thinking Incineroar for now, but thanks for answering, appreciated the reasoning.