I'm going with Penelope Clearwater. The competent, sober, older-Ravenclaw-prefect role could have been just about anyone (thus fulfilling the "important character who's gender I could choose" requirement), but Penelope was chosen (thus the "make her female" requirement). She hasn't done anything outstanding yet, thus the "apparent until later".

I've had strong suspicions for a while that both Penelope and Goyle will have awesome roles to play in the future.