I like the idea for the clicking the spell and getting a pop up on what level to cast it at. As stated, it already exists in the game, so it can be implemented rather easily.
I can see the appeal, but on the other hand its more clicks - casting hex is already annoying, especially since penalty to a particular ability roll in combat is near irrelevant as far as I know. personally, 2 buttons on quickbar, 1 for spell at default level, 1 for selecting a cast at a higher level would be nice. Combine that with quickbar buttons to open spellbook at a particular level would be nice too imo - then quickbar spells could be slimmed down to the frequent cast/always buff ones + spellbook opening ones.
