Compensation for mages for the loss of DFG should be extremely tiny and almost unnoticable
Also they should probably rework the playstyle of most of them like they did to Ahri since most of the people who use it well are cancer but that's beside the point.
Basically, since they're removing DFG, mages are going to build a different item instead. Any large compensation will essentially be giving the mage a 7th item and saving them the gold.
The compensation should be extremely small, maybe a .1 or .2 ratio increase somewhere or probably some minor base damage increase since most of the people who build it rely on it more early-game than late-game since you can't just assume that they're now with only 5 slots and are down 120 ap/10% cdr/the active; this isn't Lichbane where they just compensated the removed ratio, the item was removed and compensation should not be anything large at all.