With Riot planning on changing Rylai's this preseason...

From a post where Meddler was commenting on Rylai's potential changes:

General problem statement is that it's too universal. Having all your abilities slow is something that should be a deliberate choice you opt into, and make trade offs for, not just a default item on so many AP champs.

Later:

It might also mean we'd want to make other slowing items (Dead Man's Plate, Randuin's Omen or GLP) better options for champs no longer able to use Rylai's well.

Since the big issue with Rylai's is that - as Meddler stated - the item is too universally used due to it's health (which everyone can use), high AP (which all mages want), and the slowing effect (which works with every ability all the time, so why wouldn't you want it?); why not try putting Rylai's slow on a "charge up" system similar to Luden's Echo, Statikk Shiv, and Dead Man's Plate?

The idea being that, as you move and/or cast spells (or some other action), you build up Rylai's charges. These could be used following the Luden's/Shiv pattern - once you reach 100 charges, your next ability slows - or following the Dead Man's Plate pattern - the more charges you have, the more your next ability will slow.

Having just one ability slow may not have enough impact to make the item worth the cost, though, so it could potentially be something along the lines of "Once 100 charges are reached, your next ability used will consume the charges and for 5 seconds your abilities will apply a slow."

This would help make Rylai's more of a niche item for certain champions who can use that window of time well, or who can build up the charges quickly - rather than every champion with a decent AP ratio wanting it. It also makes the slow more healthy, since you won't get perma-slowed by any champion with low enough cooldowns - there will be windows where a champion can use the slow, and they will have to think carefully about how and when they use their abilities - do they use the ability now for the damage or to clear a wave? Or do they wait until a fight is about to start so they can use Rylai's slow? Or can they get enough charges back before the fight starts to clear the wave now?

Just an idea I came up with while reading the discussion on Rylai's.