The idea of her being 'Talisa Hill', with a Lannister father (Gerion? They could tweak the timeline so he want to Valyria via Volantis 20-odd years earlier rather than the dozen or so in the books) and a Volantene mother could make sense. She could have gone to the Westerlands at some point, met her family, had them not be particularly impressed, and she ends up as a field medic (or equivalent), which may be a reasonable job for a bastard daughter.

This opens up a potentially cliched Romeo and Juliet thing (Robb falling for someone related to the opposing family) but it has some intriguing possibilities with Jaime perhaps knowing her, and the Freys going even more apoleptic when they find that they have been betrayed for a bastard.