How to EASILY make Mordekaiser viable
I'm surprised it hasn't been done yet. Morde's biggest weakness is his lack of CC (also lack of mobility). E gave him a movement speed mechanic that when used correctly is actually fairly useful. What he needs is CC. He is the MASTER of Magnetism.
The easiest buff to increase viability, MAKE his E a small vortex knock-up. A mini Darius pull OR Thresh flay. When used skillfully it will disrupt channeling spells and enemy casts. It fits the lore and gives Morde some utility.