Hmmm... here's an interesting thought - what if Original Sin is a trial run for the improved-over-BD two-player character concept, to see if it works with new characters in a side story instead of plunging that idea into Divinity 3 untried?
That thought crossed my mind too. I wonder how it would work if they had the Dragon Knight and the Divine as the two characters in Divinity 3?
If the Divine is the main character in Divinity 3 I can't Larian forgetting the Dragon Knight. I mean his story isn't quite finished yet and they could include him in the game's plot even if he / she is a computer controlled NPC lending a hand (or wing!) whenever needed.