Possible Karthus W Change?

Alright, so personally besides his W, Karthus balance-wise is in a decent spot, His Q is his Bread and Butter ability and it's perfectly fine. His E does what it's supposed to in being able to refuel your Qs, and his R will either kill dudes, or make them waste their anti-Karthus ability so someone else might be able to get them.

His W however, I feel might be the weakest link in his kit. While it does give sight at the ends, all it does is add a slow and some MR shred and that's just about it. It feels especially weak after the ghost nerf that made the slow on his wall decay after a second. The problem with this is that Karthus's W is his major form of initiation, and it doesn't feel like it's being as useful as it could be. This is especially true when facing assassins with dashes out the Wazoo and can just go all up in yer face anyway.

So I'm suggesting something simple. Something that will help him with assassins, yet not hinder anyone else that doesn't have a dash involved. He doesn't need a freakin paragraph of text behind it, or some sort of thing that gives move speed blah blah blah. Just give him the "Grounded" Debuff that lasts as long as the full slow on the W before it decays to whatever it decays down to. (I think half?) While this does pretty much nothing against most other mid laners, this will definitely help him out with the Assassins that can burst him down in the early game.

Karthus has never really had a niche in game besides doing "Tons of Damage", but I feel like Karthus could actually do a bit more to benefit his allies and fight against assassins.