[@ Riot] Is Rylai's a support item?
For some mages, this item provides too much utility in addition to combat effectiveness. This makes me think that perhaps the problem with Rylai's is it has to much emphasis on damage. Instead of nerfing the slow or increasing the cost, have you considered significantly reducing the AP and dropping the cost? Perhaps it would function better as a supportish item in the 2500g range. Mages could still buy it but would have to sacrifice damage for the slow.
This is all very selfish of course. I am a support main who loves to play non-traditional supports and Rylai's helps me add some extra utility to otherwise lacking kits.