Karthus Slight Tweak?

Halo, this is my first post ever, so sorry if I am posting in the wrong place. (With the upcoming mages update I was surprised he was ignored).

I am of the personal opinion that ever since Karthus's Ultimate got really nerfed, he vanished and faded at least in comparison to all the other AP champions, because his main strength was his Ultimate. His kit is good, but it pales in comparison to the rest.

Here is what I think might at least jolt him a bit. He can be mobile during his passive, and if he uses his ult during his passive, the duration of the passive extends to allow casting it, so basically that's a slight advantage to using the ult while 'dead'.

Another Idea, maybe give him a mechanic that his ult cooldown is fragmented (like Jhin sort of), so that if you want to cast it on individual targets it is a shorter CD, but then it further delays it being castable on a team fight setting. (I don't like this idea, because it is probably hard to program, and...I think Riot just hates the "hey Karthus press R", besides...single ray of doom is not as cool as lots of rays of doom.

I just think there needs to be something that pushes his lack of mobility and -crazy- CC, and needing to be tanky through items, needs lots of mana, but yet needs lots of AP, and if you don't build CDR your ult almost doesn't exist and then your skill shot is conditional, delayed and has a tight zone I mean...like...come on. Why would I pick him, when I have so much better options?.