Here's your question for the night: No question it's the monthly abandoned idea dump!
If you have an idea that you like, but you don't think is right for you, put it here and, you never know, someone might pick it up.
My idea, for this month, is this: There's a (from what I gather) little known X-Men dropped plot point from ages ago.
At some point in time, Rogue attempted to absorb Mephisto via a kiss, that kiss ended up giving Rogue his ability to take people's souls (for Mephisto) at the cost of her own soul.
There was never anything stating that she got her soul back from Mephisto, so the idea is that Mephisto has let her be a sleeper agent on his behalf and now he's ready to call her in and make her his.