Here's a way you could help a whole lot of champions in one fell swoop:

Make Rylai's slow stack with champion slows again (only the one who bought it). For those of you who don't know how that used to work, it stacked multiplicatively. Meaning a 40% slow and a 20% slow create a 52% slow (0.6 x 0.8 = 0.48). The stronger the slows, the lesser the gain. An 80% slow and a 20% slow create an 84% slow.

Here's a list of all champions who are likely to benefit from this (if only in tiny ways or with weird builds):

If you look at this list, you'll see some champions who probably shouldn't get a buff like this. Champions who are already strong, popular, and build Rylai's. Others probably wouldn't even notice, champions who still wouldn't buy Rylai's or champions who only one have slow that's situational or on a really long cooldown. Some of them would only notice when they troll innovate, like Support Ashe. The rest of them are champions who are struggling at the moment and could use all the help they can get.

That way, Rylai's wouldn't only be bought by champions who've abused it in the past or need it to function at all anymore. It'd be bought by about 35 more champions who just want decent itemization.