I have a few different ideas. I would really love to find someone who would play Robb.
The northern army has been at war with the south for months now in a fight to win their independence as well as for the Starks to get their beloved sisters and daughters back. Each and every battle the Lannisters have said to have been defeated. What would happen if the King in the North gained another advantage when they would take over a stronghold and come across a Lannister. Not only that but a female? Will they use her as a bargaining chip since the Kingslayer is much to dangerous to let free, or will other things occur? And what happens if it turns out she might have information useful to the northern army?
When Lady Catelyn and her son Robb made a deal with the Frey's, most of the men joined the northern army in the march south for war. In this I'm thinking one female, a granddaughter of Lord Frey, would sneakily go to join them. Perhaps tired of being at The Twins where there was to much family around to ever really stand out in the crowd. Perhaps passing off as a commoner or bastard and helping with the wounded, what would happen if she and the king caught each others fancy? Better yet, what if he had no clue she was a Frey? Would he be considered going back on his word by wanting a granddaughter instead of a daughter?
In an attempt to make peace with the Starks; Tyrion and Cercei arrange to have one of their cousins be betrothed to Robb. Unknown to Robb, or even the Imp, Queen Cercei plans to have the cousin marrry him, birth an heir for Winterfell, maybe more than one kid because bad things happen sometimes, and then have him killed that way the child will grow up in Lannister influences and they will have yet another place that the lions rule.
Okay so this one was sort of inspired by the song The Scientist by Coldplay. Like some of the others it does have to do with a Lannister and also becoming captive. But in this one I was thinking the way her and Robb meet was during the point when everyone was at Winterfell back in the very beginning of the series. And for one reason or another she decides to sneakily stay after the Lannisters and Ned, Arya, and Sansa head back south.
I left it vague so it could be fleshed out a bit. Basically my thought was the drama and tension that would come once all of the shit hits the fan in the south and the north goes to war with them. Especially if they kind of like each other by the point she would have to be considered a hostage.
My thought for this is kind of rough and needs some work. But the idea for this is that my oc would be a Targaryen. Perhaps she could be a fraternal twin of Dany? Or maybe she's just slightly older (and maybe Dany was still given to Drogo because of how usually it's the two eldest that marry in the Targaryen's sick incest thing). Or she could be a daughter of the eldest son Prince Rhaegar who had died.
In any event I had one of two thoughts. Either the oc could have ended up left behind and a family who stayed loyal to the dragons hid her away because they knew if not otherwise, the Lannisters or Baratheons would kill her. Perhaps it's someone that is now, in current day, sworn to house Lannister, and it just happens to be the city that the northern army overtakes.
The other way I was thinking was the oc did in fact go with Viserys and Dany to the Free Cities and have been with them the whole time. After the dragons hatch maybe Dany, needing a way to start gaining support for the throne, asks my oc to go to Westeros and get in good with the Starks, seeing as they are trying to take down Joffrey/the Lannisters.
I haven't yet decided if the oc should have a dragon or be involved in that aspect in any way. But I haven't yet thought of another reason why the Starks would find her useful, other than perhaps using her in their cause and claim 'she's the rightful heir to the south'. The ideas rough, like I said, but I'd really love to hash it out more in detail.
In this idea Margaery and Renly would have consummated their marriage before his death. But the thought was, after his death thanks to Stannis, Loras wants revenge and the Tyrell's need a new alliance. With the northern army winning they decide to try and forge one with the Starks using the next eldest girl in the line after Margaery. Whether we leave it that Robb made his oath to the Freys or not doesn't bother me. (Could just add a bit of drama into it all if we left it like that.)
I'm thinking either doing something with the idea above or keep things how they went in the series and have an oc somehow captured by the army.