Karen Chen is just about the only one who (pretty much) has her ticket stamped. ETA: Ashley too -- forgot about her for a moment. *face palm*
USFS has been pushing Mirai, and really hyping her up, but her inconsistency is concerning. If she places in the top 3 at Nats, it's fairly likely they will send her, but a ratified, non-UR 3a internationally would help.
Bell has a shot, if she delivers.
Zhang seems like a wild card and for me she is a sentimental fav, but she would have to REALLY deliver.