Hmm, well, OK, I guess Shirou or Rin makes sense next, not sure which. I'm going to say Rin.
For her, I would like development such that she becomes more in touch with her human side, and in particular for her to reconcile her feelings for Sakura and openly accept her as her little sister. Fighting-wise I'm not sure there is much to develop, at least at this stage. She's already pretty competent, and I don't think we should start throwing the Jewelled Sword in here.