Allow Karthus to Q while moving?

He doesn't need it. Karthus is the most complex Mage in the game since his Q is all about positioning and reading your opponents movement. It'll make Karthus too easy to use and take the fun out of how challenging and rewarding it is to land his Q.

It'd be a rather fair compensation for how immobile he is. He might not need it, now that assassins are starting to drop from their dominance, but it wouldn't be completely unfair to give him some minor chance to actually outplay a tower diving assassin.

It would make him absolutely broken. The spell has great damage output. He's pretty strong now IMO