They should replace Thornmail's damage return

With an effect that's just "Enemy champions cannot heal off of damage they deal to you". It would no longer create the really stupid situation that old thornmail created where you just couldn't attack someone since they had built it, and it would also provide a real counter to lifesteal for tanks.

If you want the damage return, add an active that gives a really large amount of damage return for a few seconds. That way you can actually play around it.

Overall, this would be a buff against targets with any LS item and a nerf against targets without. Seems pretty fair.