Just a few ideas I got, maybe someone wants to pick up on those. Barely fleshed out, but it should give you a gist of what I'm thinking of.
The Devil's Temptation
A zealot priest. A succubus sent from hell. Will the demon be able to corrupt the zealot, using its skills in seduction, without showing its true nature? Should the demon be unable to make the priest performing things against his faith it will be tormented in the deepest abyss of hell...
I would play the male, read priest, part in this, and i'd like to have a female playing the succubus. This is all about subtle, careful seduction, to the point of commiting THE sin.
The Daemon's Temptation
It is said that not a single Sister has fallen to Chaos. The Brides of the Emperor, the Adeptus Sororitas, are the strongest bastion of faith against the corrupting influence of Chaos. But still, the daemons of Chaos are trying to get one of them. Eventually, they might succeed...
This one would take place in the Warhammer 40K setting. One would take the role of the Sororitas ( any Order, tho Sisters Militant would be most fun ), while the other would play the role of a devoted follower of the Dark Prince of Chaos, Slaneesh. Not a daemon, just a human walking the path of corruption, as any daemon would be purged in an instant.
While the role of the Sororitas would be clearly a female character, the seducer could be of any gender. Once again, subtlety is a must to make the Sororitas fall from grace. A single wrong word, and all efforts would be lost.
In this one, I'd play either. The only thing this would need is at least a bit of knowledge of the 40K universe, and the Imperial Creed.
Shoot me a PM if you're interested, or leave a post here.