That's one really cute succubus. (or demon girl, depend on her ascendance)
Really like the tail, not too big, not too extravagant, kinda gentle like a girl.
Oh, and really nice butt too.
You're also right about the horns and wings. The horns will be a great decoration for her different hairs styles. (because i know you like to chance the hair style of your characters a lot.) Beside, demon doesn't really need their wings all the time (they can hide it usualy) except when they need to fly, like the little imp next to her.
Talking about that imp, i don't know what would be your plan about the next story, but it could be a good idea of a sidekick.
Overall, that drawing is one that really moved me. All your swimsuit character does anyway. ^_^. Always thought we didn't see enought Natasha in a swimsuit.
Btw, is it finaly the return of some fanservice boobies?