It won't do to build up a fresh character new to the series and play them for 50 hours only to switch to the Divine for the end.
What is the Divine One going to do for 50 hours? Unless he is fighting entire armies, or the time in the Plane of Hypnerotomachia reset his abilities, there is not much that we've seen in Rivellon that would offer much of a fight. Of course, Damian could always unveil his secret weapon, which adds 60 levels to all the bad guys, though Rivellon would need some excellent new defences not to be wiped out by anything that could pose a challenge to the Divine.
I'd be fine with Damian and the Divine being stalemated, with the player character having to do something (like rebuild the Council of Seven) to tip the balance (possibly in either direction).
That does make a certain amount of sense, they could do something like that. But I'm not sure that would make for the most enjoyable boss fight. If the player's main contribution is just hitting a few levers to activate a Damian-Zapper, or hitting Damian in the back a few times while he's occupied with the Divine, that's not exactly the most thrilling idea for a fight, now is it? You could see that for yourself in Divinity 2 by using your creature/Demon on some boss fights to distract the bosses, or in the fights in Laiken's tower when you have Sassan.
Depowered by the plane of whichever over the course of 50 years could work as an excuse to have some undercover low-level questing as the Divine recovers his strength, but I'm not sure what excuse the game could give if they wanted to use the DKS Dragon Knight as a player character, as they were crazy-powerful at the end of DKS (all my character ran out of places to put their tons of skill points).
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"Importing the saves" is a nice idea in theory, but at best, that wouldn't really provide more than a token benefit. It's too resource intensive to balance the game twice for completely new characters and imported high-level characters. How many players really would be able to use an old character? It was mostly done in Mass Effect because of the story consequences of your decisions.
Larian didn't even bother to carry over story threads from Ego Dragonis to Flames of Vengeance on the same playthrough of DKS. It didn't matter if you sided with Antumbra the assassin, or let Jackal go free, or didn't free Folo's pigs, or even if you never gave Willy a single coin - Antumbra and Jackal never showed up in Aleroth, Folo and Kevin did, and the game thinks that you did pay Willy some money (if you mind-read Willy in FoV). If they didn't do that for the same character in the same game, I don't think Larian is going to carry over saves for another game coming out three years after Divinity 2.
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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?
