IMO there's nothing wrong with Minfilia as a character, just the presentation of it. She coordinated and lead the Scions from their formation up until Ul'dah, but very little of the importance of her position was shown beyond giving orders.

Yda's probably the Scion I like the least, though my immediate reaction to a character I dislike isn't the desire to see them killed off. It'd probably be Thancred or Tataru, as their deaths(real deaths, not just MIA) would carry a lot of emotional weight.