The character in Beyond Divinity was able to defeat Damian in battle. Perhaps he/she can also come back in Divinity III. The player character from Beyond Divinity, the Divine One, the Dragon Knight in DKS, and Zandalor can work together collectively in fighting against Damian and his forces.
Well, you do have to win that fight (dying is still game over, Kein), but after you "win", Damian says that he was just playing with you and throws you into a rift going somewhere, so I'm not really sure that counts as "defeating Damian", because you don't. Damian wins no matter the outcome.
I'm not really sure about a "kitchen sink" final battle idea. We've already got people who didn't play Divinity 1, and how many never played BD? Throwing in too many references might just be confusing.
Also, the BD paladin would be like 70 years old by the time of Divinity 3, and unlike the Divine or Dragon Knight, he/she wasn't bestowed with a long lifespan as far as I know. (the 90+ year old Seth and Baud and Deodatus in BD were stretching things enough.)
