As for Divinity 3, I don't think people would refuse to believe Lucian was the Divine One, but he would probably need to loose his powers if he was the main character (to make it easier to balance / design the game, and allow for a different skill system, etc). An illusionist could take care of the character customization.
That's an interesting idea, actually. The concept of illusionists existing in-game actually lends quite a bit of flexibility. Perhaps at the start of the game, because he is weak and depowered temporarily, he has to disguise himself with an illusionist for a while until he can regain his powers, after that, he can resume his real appearance?
He may be relegated to support staff (like Zandalor), perhaps using all his powers to counter Damian, while the player characters goes off on a quest to tip the balance of power.
An interesting premise would be to re-assemble the council of seven again. The player could track down the remaining elves, dwarves, etc, who were either in hiding, or escaped Rivellon with the help of rebel Raanaar, or something.
This idea is interesting, but if the Divine One does return, I don't really like the idea of having him either sit on the sidelines while the player does all the work, or he goes off to do the IMPORTANT stuff like fighting Damian while the player character acts as his coffee-fetcher.
Divine Divinity and Ego Draconis both established that the incarnation of the Chaos Lord (which is what Damian is - is so incredibly powerful that no mortal can defeat him. That's why the Marked one needed to become the Divine One - so he could defeat the Demon of Lies (who also had a piece of a god inside him). Ego Draconis has you trying to kill Damian through the soul-forge because you couldn't do it yourself.
I don't think that Divinity 3 would really work that well if the objective is to kill Damian, but you can't do it because you're not the Divine One. Also, as I said, it would be more fitting and hopefully more emotionally moving if Lucian is the one who has to kill Damian, even though he considers him his son. If it's just "Insert Name Here" Hero who kills Damian, you lose that emotional connection.
The idea of gathering another Council of Seven is an interesting one, considering how much worse the world is now, what with Elves apparently going extinct, Dwarves in hiding (or also dead), Orcs as slaves to the Black Ring, Imps hostile, and Lizards missing. But it might be kinda repetitive of Divinity 1 if you had to do that again, depending on how it was handled.
