Mordekaiser has 1 part of his kit that holds him back.

So, as you know, Champions have a... what did they call it?... a cost on their kit? Something like a built in revive takes up a lot of that kit cost. So you could imagine something that gives bonus exp would be pretty costly too. Morde has that, he gets bonus exp if he kills a minnion with an ability if an allied champion is nearby. This was given to him to make him viable in bot lane, but he's still not viable in bot lane, and that bonus exp mechanic is cutting into his kit cost quite a bit. If riot were to remove that, and give him, say, added attack range and lifesteal on Q, or like 10-50(by level) more movement speed, he would be much more viable top where most people play him anyways. He might even become a champ that you don't need to main or one-trick to do well with. Hell, people might actually ban him then.