Morde Synergy
Frankly once they remove Rylais it will be safe to say that Mordekaiser will have the worst Synergy with the framework of League. Aside from Rylais the items he syngizes with work better on every other champion who buy them.
The runes Morde can choose from are multitudinous but with his abysmal base stats lots of which provide glaring weaknesses: horrible base health, attack speed, and Movement speed
All limit your options to either :double down on his strengths go full damage or attempt to cover some of the base state weakness and cripple your own damage.
Dear riot Rework him soon or you lose another long time player