this is another idea i had posted a wile back for a Karthus update.
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 1,2,3,4,5 per cent spell vamp
Active when toggle on defiled he gets bonus % of ability power to all abilities you toggle off defile. You then loose all stacks and you loose all stacks at the end of death defied.
I think this would be a nice revamp to an old champ and would allow him to return to the pro seen. I would help to prevent early game magic resist shut down and help give karthus back some damage. I would increase his skill cap suitability and would not make him over powered. with max stacks and having defiled on you can have 55%/75% Q 55% E and 75% R This could be redused to only 0.5, 1, 2, 3, 4 Q. I know the ult seems op but late game if it doesn't kill someone life steel usually negate the damage.
I would also like to see 2,4,6,8,5 spellvamp per-stack and 1,2,3,4,5% bonus ap to all ablities. Since you only gain stackes when defile is toggled off and loos all stacks when toggled from on to off, this max for a fun mechanic.
Max stack 200ap change lv 1-5
Wall 5%=10 10%=20 15%=30 25%=50
Defile 25%=50 30%=60 35%=70 40%=80 45%=90
Lay waist 35%=70 40%=80 45%=90 50%=10 55%=110
Lay Waist single target and ult 65%=130 70%=140 75%=150 80%=160 85%=170
Max stack 1070ap change from lv 1-5
Wall 5%=535 10%=107 15%=160 25%=267
Defile 25%=267 30%=321 35%=375 40%=428 45%=482
Lay waist 35%=375 40%=428 45%=482 50%=535 55%=588
Lay waist single target and ult 65%=695 70%=749 75%=803 80%=856 85%=909
%ap could be change to .5,1,2,3,4
Or they could add DFG's effect to his wall of pain and give it an area of affect that if an enemy champion would dash, flash, or tp in the area of the wall excluding ult they would instantly get teleported in to the wall and stunned for 1.5 sec.
Similar to how has a wall that stops the dash this one redirects you in to the wall giving Karthus more depth. I could even be renamed Torn Vale of Eternity
You know he is warping the space between life and death thus DFG's effect on wall would make seance as would the warping effects on dashes and tp's.
they could even change his Q's so they deal double damage large jungle mobs and champs as aoe damage and triple damage to single target large jungle mobs and champions.
NOW
Lay Waste
Cost: 20/26/32/38/44 Mana
Range: 875
Karthus unleashes a delayed blast at a location, dealing damage to nearby enemies.
Creates a delayed blast at Karthus' cursor position. After 0.5 seconds, deals 40/60/80/100/120 (+30% Ability Power) magic damage to each nearby enemy, this deals double damage if it hits only a single unit.
Patched
40/60/80/100/120 (+30% Ability Power) aoe minions
80/120/160/200/240 (+60% Ability Power) single minion and aoe to champs / large jungle mobs
120/180/240/300/360 (+90% Ability Power) single target champion and large jungle mobs.
this would do a lot of good and would keep him from out farming but give him that early game damage he needs.
(or on rare occurrences against one) I ult with a certain degree of safety to kill someone. If i ult with lvl 1 ult i wait until they are between 250-150 health just to be safe and not waste my ult. The problem is that sometimes they might live because of their resistances. 
gets a visual indicator for when his Ult will kill so can 
