I think I might have a slight idea to make Freeze a bit more valid. Taking some inspiration from the recent SMN changes to Tri-Bind; cause that's another issue, lol. But why not add a trait that makes Freeze act like Flare but instead of using all your MP, you have to have no MP. For instance, you do your Fire rotation and end in Flare, you have no MP left and no way to get it back but to use transpose to enter Umbral Ice. But with a new trait, casting Freeze would allow to enter Umbral Ice and continue using your AoE on the mobs. This would avoid the odd second or too between MP ticks that sometimes can occur. It's just a thought. Freeze doesn't have to get a boost in power or anything like that it just allows you to maintain your rotation easier and gives Freeze a use. Maybe make it level 54?
a mock-up
Sub Zero Lv. 54 When MP is reduced to zero, the next cast of Freeze will cost no MP and grant Umbral Ice III
