Never say never. The series with Ash as a main protagonist will most likely never end. It's disappointing, but true.
I think the only thing we can rule out are legendaries. Even then, there may be a point, a dozen seasons from now where he befriends on to the point he uses a pokeball.
...although I suppose it's safe to rule-out things like Koffing, Ekans, and Meowth.