Well part of the Divine One's quest could involve reactivating his abilities. Being trapped in that plane for 50 years could have made his abilities fade as Stabbey said. Damien himself in Beyond Divinity started out at LV 1 despite the fact that he was supposed to be rather powerful by that time. The game could also give The Divine One a few level one abilities (you know the Divine abilities) just to show that The Divine One truly is the Divine One.
That still doesn't answer if it's set in stone if the The Divine One will now have the name of Lucien and be a male. The Child of Chaos novel made The Divine One a male called Lucien by cannon but Larian could easily pull a Bioware - let's say - and have Lucien as the default name which the player can change if they wish along with gender. So those concerned about cannon can play as the default Divine One who has a set and iconic appearance whereas others can create a custom character.
I wonder if the game will even feature character customization in terms of appearance. It's top down right? I'd imagine they might allow for Baldur's Gate or NWN type customization where you can choose skin/hair color and choose from a set of faces or they might just make it like Divine Divinity with the three different models and appearances for each class and gender.
Story-wise it would rather make sense for the player to resume the role of The Divine One I guess. The story wouldn't have that much power to it if you're a new guy simply fighting against The Black Ring while hearing about The Divine One doing all the big stuff and eventually defeating Damien alone.
For all we know though, the game could be party based with The Dragon Knight and Divine One teamed up. The DKS could still turn into a dragon but it'll just work as a shapeshift spell if anything. I'd prefer that actually. Hell, fast travel could consist of The Dragon Knight carrying The Divine One to places.
