My idea is kind of weird. It has a mix of romance and family drama all in one. Basically previous to the rp, Steve Rodgers met and fell for a girl who was a villain. She 'turned herself around' and seemed to be actually wanting to do good. Two years after they started dating, the two were finally intimate and after finding out she was pregnant, the villain girl left him without a trace. As it turns out, the girl was still working for a secret organization who wanted to make an even better super solider. And, in order to do that, they figured they would breed a person who already had powers with the super solider to get an even better super solider. The girl's whole job was to basically sleep with Steve until she got pregnant. Steve had no idea about any of this and in fact, doesn't even know where girl went off to, much less that he has a daughter.
Years pass and the daughter has basically lived her life in this organization's lab, learning to make her powers stronger and being a live test subject. Finally having enough of it all, she breaks out of the labs and they try to chase her down. Though, in the mix, the organization make her seem like the bad person even to the point where now shield is out to catch her, sending the Avengers to get her.
Basically its a mix of Steve finding out this girl is his daughter and trying to form a daughter-father bond with her even though she wants nothing to do with him, a romance between the daughter and the son of either Tony, Thor, Bruce or Loki, and maybe even a possible romance between Steve and his past lover depending on how we make the story go.