I have to admit that I would be very surprised if the game never gets any expansions at all. It's already been too successful for that not to be in the cards unless Swen played hardball with WoTC and put something about never doing any expansions at all into the contract--something we can't know as the agreement is clearly under an NDA. At the same time, the only expansions I'm currently expecting are things like the "Rum Runners Pack" from Pillars II Deadfire. We might get an extra companion or a couple more hirelings--possibly another background option. Maybe some items or a new vendor. Just a little bit of free fluff to keep the game in the public eye.
A major paid expansion is something I think is less likely, but possible. Maybe some sort of quest in Elturel or the surrounding area that ties into the current game. I can't see such an expansion adding many more levels though. The furthest I can see them going is maybe 14th level, as 5E really starts breaking down above that. Anything in the 17-20 range is completely out in my opinion, and previous editions of D&D are a terrible example for thinking of what they can do with the 5E level cap. IF they do such an expansion I would expect it to take at least a couple or three years even if it's much smaller. After all, you need 100,000 XP to reach 12th level, and a further 40,000 XP to reach 14th. That's going to take a lot of content to provide.
I would love a major expansion, but I don't really expect one. However, if the game gets GOTY and tremendous sales, I would be less surprised as WoTC will be really pushing for it and the agreement could give them the whip hand in those negotiations--though then again it might not.
