Often times people get Banshee's veil to help negate against heavy caster teams... I was thinking maybe a similar item for casters against heavy physical DPS teams.
Basically, I was thinking like a modified version of the Thornmail and Warden's Mail combined... like a High defense item which not only returns damage, but it should return damage based on a % of your total AP... This way it wouldn't really appeal to regular tanks because the returned damage would basically be so minor it'd be a waste of gold unless you had a decent amount of ability power. Instead of "slowing" the attacker's movement speed and attack speed, i'd suggest a 1 second stun or something not sure what the internal cool down should be...
This is just an idea, please give me legit feedback people
-Returns Damage based on % of AP
-proc'd mini stun
Basically an item for those longer games, wouldn't be an item you'd rush as a caster.