Some minor Karthus changes that would benefit him greatly
Title. As the first non-450 ip champion I ever bought, and my mid-lane main since I started in S4, I believe Karthus needs a little help to get him out of the spot he is now. Of course, overtuning him is definitely something nobody wants to do. So, here are a few changes that I think wouldn't make him too powerful:
Q: Karthus feels sluggish when he can't properly kite when every time he Qs, he stops moving for a second during the cast time/animation (NOT the delay for the actual Q). So, removing the cast time of the ability (similar to Lissandra W, where she can actually move during its cast animation) would give him just the kiting ability he needs as an immobile mage.
W: In addition to shredding MR, Karthus should be granted a portion of that MR for the duration of the spell. However, he gains slightly less MR from each additional enemy hit, to avoid abuse in teamfight. This is aimed at improving Karthus' trading ability early game (against other mages, so that assassins still counter him) so that other AP mids don't oppress him as much as they are now.
Stats: Raise his level 1 armor by 2.5-4.5 or so. Is quite self explanatory, as to help him a tad more against the AD mids that counter him.
Of course, there are plenty of other changes that Karthus could receive that would help him, but these are my preferences. What do you guys think?