I think this is a great idea. Really, anything that would move the game closer to being in line with Neverwinter Nights would allow Divinity to retain a large playerbase for years and years. Divinity already has a one over on NWN in having a great first campaign. I think the best choice for expansion would be fleshing out the modding tools and facilitating the ability to create persistent worlds. One of the most obvious ways to do that would be implementing DM tools or a separate DM client.