Karthus is missing something to make up for certain falls in the mid game. So I would like the to add a stacking passive to defiled. Passive each kill give you a stack, max 5, each stack gives 2,4,6,8,10 per cent spell vamp. Active when toggle on defiled he gets bonus 1/2/3/4/5% per stack to all of his abilities. You loos all stacks when you toggle off defile or at the end of death defied.
You gain life to negate burst damage and you deal bonus damage wile defile is on with stacks so it give you the ability to also prevent turret dives when at full stacks but makes you weaker after you have used defile and have to farm the stacks back with out using defile.
Give the wall damage over time equal to the number of stacks.
His wall could go back to scaling an extra 5% mr and armor every level. His ult could inflict 5 seconds of damage over time when you star to channel it.
Make his ult do more damage the more ail champs that are dead. Max 5 due to death death-defied.
or increase his base mana by 361 + 85 per level.