a succubus is a sexual demon that seduces men in their sleep and steals their souls
it sounds cool
so I guess your succubus character will most likely be busty and/or cute (Chi from Chobits cute) considering her "job"
and perhaps there will be a couple of succubus friends trying to stop her from her forbidden romance with the trapped angel
hilarity, ecchi, and fanservice ensues
and also a legion of angels trying to rescue him
a clash against the demons who want to use the sealed angel/demon for their own purposes
the power of holy
fighting fire with fire and all that jazz
a complex holy war just to save the "ultimate weapon" against God's armies: one of their own
dun dun dun...
you could just go with a Love-Hina/Tenchi-Muyo-type, romantic comedy
hilarious misadventures and cuteness
a bishounen (cute male anime character) character for your angel who transforms into a badass demon who is also bishounen
and of course, fanservice and ecchi from your succubus