First, i don't think he necessarily needs a change but i thought of this and thought i'd post it because it's just one problem i have with him that's a little annoying.(I don't have any other problems with him.)
Mordekaiser's R is very annoying for champions that aren't good at fighting him, such as Mages or the Squishier less-mobile ADC's, it's nearly a death sentence and even if Mordekaiser is behind he can still just press R on you to remove you, a Carry from a fight. This annoy's me because i play primarily mages and while it's not necessarily hard to run away from him, it's the fact that if we're even or he's ahead i basically have no choice but to run or die that's the most annoying.
This reminded me of when Illaoi had her E that lasted 30+ Seconds, and if i remember correctly you couldn't lower the Duration by attacking her, so needless to say it was super oppressive to have to run away from tentacles for 30+ seconds while illaoi's voice lines play in the background.
What i'm getting at here is that i think it would be Appropriate to introduce a mechanic to Mordekaiser's R that makes fighting a viable option to Carry's. As it stands being forced to hopelessly Kite away from a champion who's R is a built in lock-down stat-check is annoying, and Fighting isn't a viable option to champions that can't outright tank him, but what if the duration of the ability decreased by a certain amount after each attack?
Not saying it NEEDS to be done, but it would be very nice and probably help remove some frustration around him.