I'm still reeling about that cast. Seriously, out of all the announcements people expected for that direct, did anyone consider "Chris Pratt is Mario"? Absolutely bizarre.
I'm kind of hoping that maybe Martinet will voice Wario in the film as one of his "cameos". It feels kind of unlikely though, I agree that the movie will most likely be about establishing the central characters/story and a sequel would introduce Wario, Waluigi, ect. I also feel like Wario is a big enough character if he were in the movie he would also get a celebrity voice and probably would have been mentioned in the Direct. I like the idea of the movie beginning with a Wario and Waluigi short though, lol.
If they were to get a celebrity to voice Wario, who would you want? Most people I've seen want Danny Devito to be him (which I think would work), but given how strange the actual voice cast for the movie is there's a part of me that doubts they'd hire the obvious choice.