How to make Karthus better
On his ult, make it so that it reveals all targeted enemy champions, even camouflage/invisible ones. Champions who are immune to targeting (zhonyas or an ability such as Fizz's playful/trickster) will NOT be revealed. The reveal could be like Twisted Fates ult except maybe only apply to champions with in a certain range of Karthus (like quarter of the map diameter or a full jungle quadrant).
will be the death of his already low play rater)