Even if we get a post story expansion, I don't think we can continue questing afterwards. For example, I would like to leave NC forever, as it was possible in the Nomad ending. They would need so send you back to NC to be able to continue playing. I assume that leaving NC forever will be an option
I don't really see why they couldn't have post questing. V in Nomad ending could just be wrapping things up at home base of expansion before leaving forever. Blood and Wine doesn't stop Geralt & Triss potentially ending up settled in Kovir eventually.