Two ideas based mostly in the same theme:
Three is Trouble:
It is generally known that women who get involved in the life of a superhero, be the villainous or otherwise, tend to be abnormally well formed. Part of it may be that the rigors of such an active life keeps them fit, but it fails to explain the generous proportions that give them what is well known as the "Most common female superpower." And she was no different. So well formed as to have been able to make likely a more lucrative career modeling than she did in petty crime, he had always found her strange. As she was never truly wicked, never cruel, never evil, there was something about her that was charming. The long dashes, the struggles, they felt more like playful competitions than any real attempt to succeed in her misdeeds or defeat him. He had a strange soft spot for her, and she had quite a liking for him. Fast and strong, her match, at least, better than those she saw when she was free of her mask, when she walked in the day. She fancied him.
But things are never so simple.
For during the day, he had a girlfriend, a close one. Long had they dated, there was a closeness, a warm friendship and more there. And she knew about it. Still, she was not prepared to give up on him. Not yet.
They were planning on moving in together, though, and that represented a real threat to her plans, her lusts, her desires. She would face him again, and this time she would win him over. Win him with her looks that were so much better than... Than that woman's, with pleasure, with lust, she would overpower him if she had to, but she would let him know her feelings, let him... Feel them, feel how much better it would be to be with her.
This scenario could go a few ways. It could be a functional one shot where either he 'catches' her but she seduces him even while restrained etc, or perhaps she does overpower him and gives him a very physical tour of her body and what she is capable of. Could also be done over a longer duration, her slowly trying to inflame his lust and break down his resistance.
I have a preference for playing the female here, but who's who could certainly be played with.
Taking Whatever She Desires:
This is a more straightforward affair. More properly villainous female has found herself developing a quite distracting attraction to her arch nemesis, and is finding that her plans are foiled now almost more by her own hesitation than by his good timing or abilities. As she considers herself quite 'professional' in what she does, this drives her to no end of annoyance. She decides there's only one thing for it, to find a way to share his bed, to get in close to him, if she needs to capture him or surrender to him to do so. If it ends poorly, great, she will not have to worry about her internal conflict anymore, and if it goes well... Then perhaps she could lure him to work with her rather than against her.
Exact details of how this would play out to be worked with between me and my partner. Post here, PM me, add me on one of my IMs if you'd like to chat. If these game's aren't exactly your cup of tea, feel free to drop me a line anyways, I have lots of ideas, and am always willing to hear new ones.