I think Fallen Heroes, or at least a similar game, would have to happen at some point, if only to bridge the gap in lore between DOS2 and FoV. Otherwise, going forward without explaining these plotholes would only confuse matters.
There is also a lot of prequal potential in the Divinity Timeline. I for one, have always thought it would be neat to base a game around the formation of the Order of the Source hunters, and eventual overthrowing of Braccus Rex.