Can Karthus' Ult get a bit more early game damage or maybe a % health scaling?

I've noticed that ulting early game (like when you just hit 6) seems to not kill anyone who has even the slightest bit of sustain. 250 (+60% AP) at rank 1. Anyone low enough to justify using it seems to always be able to just heal enough to survive, or are tanky enough to simply shrug it off. Also, is really powerful for spell spammers and is taken quite often (even by karthus). This item is not hard to acquire before level 6 as well. Meaning that AP mids and other champs now have a form of regen as well. Not to mention Warlords Bloodlust, or how every adc starts with 3% lifesteal from and possibly more depending on masteries/runes and their current build in-game.

I do not think his ult was made for kill stealing. It seems like it either was intended for starting a conflict or for picking off those who escaped with low health. But with a 200 second cooldown at rank 1, it seems to have a remarkably low kill potential.

Maybe if it was given a passive which ignored some mr (only applied to the ult, not Karthus) similar to yasuo. Or add a % health (max, bonus, or missing target's max, target's bonus, or target's missing) to give it that extra boost. It is such an iconic ability which Karthus is known for (besides his passive). But he is falling behind with the times. MR is being prioritized over AP on mages ( into ) and is cheaper. So he really can't utilize his ult well unless he is potentially kill stealing another laner or if he is killing his own (which makes it being global for the full enemy team kind of a useless attribute because it is being used to only kill 1 target whom which you are directly fighting).

Edit: Another idea could be to make his passive give him a lot of AP when he dies.