I'm in Act III and I'm dragging my feet talking to every NPC possible because I'm afraid to follow through with the main story because I don't want it to end.
I would love to see expansions for additional campaigns, side plots, extending the plot, etc. I would 100% be willing to pay substantially for more content.
Most of all, I would love to see something implemented where the community can create their own campaigns (e.g. DM/GM mode). This game's mechanics are already set up perfectly as a platform for that. I would gladly pay another $60 just for a DM/GM mode. I've seen posts/articles saying that's not gonna happen, but we if the community pushes for it and Larian profits from it, maybe it could happen.