I feel his ult can be changed to give him a decent buff.
Requiem (R)
ACTIVE: Karthus loses all vision to focus and channels for 3 seconds and, upon completing the channel, he can cast requiem again within the next 5 seconds to deal magic damage to all enemy champions.
Karthus cannot move or deactivate after activating Requiem until channel ends or is interrupted.
So basically, you still get the 3 second notice of his ult, but Karthus has a 5 second window to detonate it after that. To make this a game of 2 sides, Karthus cannot see enemy champions while casting his ult. So if he's just sniping someone with no shields he will just detonate his ult as soon as he's channelled. However, if he's trying to hit one or more people who have shields or barrier or stasis, he has a window of opportunity to actually hit them by baiting it out and hitting at the end of the 5 second window, or hitting them immediately which could also catch them off guard. Another note - it is a risk to Karthus doing this as he is even more vulnerable this way while using his ult.
I think it's a quality of life buff that simply makes his ult more interesting.