Theurge is pretty solid for GvG and PvP, Dark Pole is not good enough for someone who already has Frost Cloud but it’s still good to have.
RC would be very good if you have a Cryo 3 Chrono 3 and a Wiz 3 Kino 3 in your party for example. (or any cryo kino, this is just a more optimal way to do it, maybe a monk in the place of the Kino?), also a few more useful spells (specially destruction (better than meteor) if you can 1-shot dudes).
Featherfoot has VERY good debuffs, even if their damage is not solid enough, anything with Decay gets 1-shot by Musketeers. But I don’t think many people would go for it as making other dudes kill more is not something good enough to get in the place of more damage.