I disagree. Right on the Divinity 2: FOV website, is some perfect speculation fuel.
It says that the Dragon Knight will try and free the Divine One from imprisonment. It's easy to think of a way to rationalize and say "oops, that worked, but now he is weakened and depowered" and poof, you have a player character. It would also be very fitting if the Divine One is the one who takes down Damian. It was his destiny all along.
It is also possible that Damian will be vanquished at the end of FoV, and Div 3 will be about something else entirely... because otherwise what would be the reason for the Dragon Knight to disappear at the end of FoV (which (s)he apparently will do)?
That is certainly possible, but I believe that:
(Speculation)
The one who the Dragon Knight REALLY wants revenge on is Y'Gerna. She's the one who tricked and trapped him, and she would be a suitable final boss. Damian just showed up occasionally to warn the player to "cut it out" before at most throwing a few inept guards at him.
If the Dragon Knight vanishes at the end, it might be to free the Divine, so even if the Dragon Knight dies, there will still be a sense of hope and victory because it will have real meaning. (Heck, he might even pull off a Talana with the Divine.)
