Hm, interesting. I thought Frost seemed pretty good because her slide is safe and launches the opponent, similar to Cassie b1. She also has some damn good zoning tools like her ice shards and the ice-berg impaling move.It's up to you man. If you want a ranged move that you can hitconfirm into a combo/blockstring then maybe play Superman or Green Lantern. If you want mixups then play Batgirl or Wonder Woman. I feel like you might as well not play Frost; the only thing she has over Batgirl is that she has a more damaging vortex (with meter). Frost is still pretty free to jump back and on a read or maybe read-reaction at the right distance the opponent can stuff the slide with like a normal.
What do you think about Harley? She's got high-damaging combos and guns for zoning. She can keep pressure up and mix ups with her unorthodox and weird fighting style. I tried her out and I thought she was fun, but her attacks were pretty slow.
I feel like Batgirl plays most similar to Cassie, because of her lower-damaging combos but confusing mix-ups.
Edit: How's Catwoman? She has really fast overheads and lows as long as she keeps her pressure. Plus her small hitbox sounds like people are having problems combining on her.