to sum up some suggestions as alternatives that people made in this thread:

- bonus/reduction of damage/accuracy depending on whether standing/moving while casting
or
- slow down the mages for the casting time
and/or
- adjust resistances in PvP

and a nerf in mages gameplay might need crowd control spells / instant spells / debuffs as compensation.
(in the same update when the nerf is done - not in 2.0)