Karthus Ult diversion

Just a thought I had the other day. What if at Level 6 (Ult Level 1) you could choose one of two paths for Karthus Ult? You had to stick with it for the entirety of the game.

Option 1) PvP Damage. Basically, this is exactly his ult now. Option 2) PvE Damage. Here, Karthus ult would no longer damage enemy champions. Instead it would only damage enemy minions. Damage dealt and whatnot would obviously need to be adjusted. For this option, it may be interesting to have his PvE ult range limited. So Have Level 1 damage a smallish area, final level covering entire map.

I think it would make some interesting situations. Max level karthus ult with full build can wipe out all the enemy minions from the map.