Gate the slow on Rylai's Septer?

Why not just make it an item that goes on cool down after a spell has been cast?

Ex. Brand casts his pillar while he has Rylai's, slowing all enemies. Now that Brands pillar has been cast, Rylai's now goes on cool down for X seconds until the next spell he casts will have the additional slow again.

This would fix the problem with the abuse of permaslows while giving more room for its power budget (more hp, ap, or slow potential).