Have you guys already thought something about Elves who own one of the Three Rings while Sauron holds the One and voluntarily serve him?
Just like Maeglin in the Silmarillion swears fealty to Morgoth in exchange of the realm of Gondolin and Idril Celebrimbral (Elrond's grandmother xD), an Elf from the Third Age could swear/be forced into allegiance to Sauron and in exchange be given one of the Three and a realm on top.
Improbable, I know, but it seems to me just the kind of thing Sauron would want to pull off (since we already know he'd fancy some elven minions...).
Interesting idea, i add this on my to-do list !
I suppose that if a character got some bad traits (like cruel, zealous, envious, ...), an event can pop up with an offer from Sauron, sounds good!
For the holders themselves (Cirdan/Gandalf - Elrond - Galadriel) i'm not sure, they're "good" characters and i can't see them accept an offer from Sauron, even from a crown or something like this?