So this is how I would re-work Rylai's Crystal Scepter
"Hitting an enemy champion with non-periodic spell damage slows the target by 40% for 2 seconds. This effect cannot occur on the same champion more than once every 4 seconds."
>It's not abusive on DOT mages. >It's not abusive on low CD mages. >It's not useless on other kite mages, since many of them are not looking to toss out spells much more often than 4 seconds.
Do the stats need tuning? Mm, probably.
100 AP 300 HP
Drop the price by an appropriate amount.