Hmm. Basic Succubus Fiend of Possession is a pretty simple and obvious build. ExisD could probably realize that 'Fallen Angel' character idea with any number of good-aligned outsiders, now, though the one that springs most readily to mind for me is to use the 12-HD Astral Deva. I could try a monster with some form of innate spellcasting or manifesting, or maybe one with a boatload of SLAs...
...Darn, the Beholder doesn't have a listed LA. >>; Oh well. Maybe a Silthilar? Nah, Polymorph Any Object at will, even as a full-round action, is probably going to be a little too overpowered, while simultaneously pigeonholing my role to one very limited in scope.
Lessee, no real undead that catch my interest, Night Hags don't have a listed LA... Ah, but that's interesting. Efreeti don't have a listed LA in the Monster Manual, but were given a +9 LA in Savage Species. And they have 10 racial HD.
...Nah, there's no way Autocad would allow that. Oh well. Succubus it is.