Would be cool legendary skin! Imagine everybody getting struck by lightning when he ults?! So cool! And electric static field around him when he uses E, a wall of electricity, throwing lightning? And he turns into a solid plasma with electric field around it when he dies. Full Metal Karthus (or PROJECT: Karthus, whatever, but I really love an idea about robotic Karthus skin)?
Imagine when he dies his body (parts of metal) could explode to pieces (kinda like Xerath's) and drop on a ground (right now it's not the case, he just turns into a ghost without even leaving any kind of body) but the electricity what runs him is still active (his passive). And then his plasma slowly becomes more and more transparent and static field around him becomes less intense, as his passive runs out of time, until his energy dissipates (right now it's not the case, he just vanishes instantly once passive runs out). That would be amazing!