Larian are making "The RPG to dwarf them all" after they have done with Dragon Commander and Original Sin - sales of these games will fund this next RPG. Here's a link that explains it
http://www.lar.net/2012/01/16/the-route-to-the-very-big-rpg-that-will-dwarf-them-all/It does not say that this next RPG is going to be Divinity 3 or not but I agree with you and think that it should follow on from where DKS left off. Although the story was concluded at the end of DKS it does leave it open for a potential sequel as Damian's army is decimated but not totally defeated & Damian himself is still alive.
I have a feeling that Dragon Commander was originally an experiment to see how the dragon combat could be improved in Divinity 3. This experiment / concept grew into a full game & Larian decided to withold the Divinity 3 project for a while and release DC along with Original Sin (Project E) first. Especially seem as they are self publishing.
So I'm fairly confident that Divinity 3 will come along eventually.
