I think thematically it would make more sense if Mordekaiser was an AD champion

I know what the logic behind making him AP in the first place was, mainly "oh he uses dark magics, so OBVIOUSLY he should buy what all mages buy!". And that kinda made sense in season 1 I guess, but let me explain something to you.

But before that, a question: who here knows the show Hellsing:Ultimate? It's some gore-crazy slightly-over-the-top action series mostly centered around vampires. Quite in the beginning there is a conversation to "why should you fear the vampires?". Is it their ability to hide in the dark? Their power to manipulate blood? Their control over rats and bats? Tjheir insane regenerative capabilities? All those are fearsome tricks, but all of them can be taken care of with the proper precautions and preperation. There is just one thing you cannot prepare for: for how FREAKING STRONG the monsters are. Even if you neutralize all their tricks and special abilities, you still have to worry about the vampire just walking up to you and tearing your head off with one hand.

And that's what Mordekaiser is. Yes it is true it seems impossible to kill him and that he controls the mist as well as the undead that dwell within - but that's not your problem when facing off against him. The main problem is that he probably won't NEED any of his "tricks" as he will just crush you with sheer size and physical power alone.

Morekaiser's archetype is not the "Undead Overlord" or "Lich-King". He is, or rather was, pretty much a "barbarian king" - the strongest the world had ever seen - who now just HAPPENS to be undead. And you don't get to be the "barbarian king" by birthright or smarts - you get it by being the strongest, meanest, and most cruel in an entire culture revolving around strenght (you know, the ancient culture freaking NOXUS emerged from). That's also the point where he's suddenly not a simple villain anymore: in his worldview and cultural moral views conquest and domination are just natural, and if one wages war that doesn't make them "evil", but a completly normal human being utilizing the strenght he has accquired to change the world around him to his liking.

We have just seen it in the Pentakill video - the dude's HUGE. No, he might appear like that because the yordles are so tiny, but he's pretty much twice as big as Yorick. Given that he's hunched over I can estimate that the man is about three to four meters in size. Now, would such a man, of such size and with such build, would even bother with magic when going to battle? Barely, in most cases he'd just crush your skull with his bare hands, and for the harder cases he still got his weapon, a mace that most humans probably couldn't even life because if its weight.

So yeah, if you rework him, consider making him a proper Juggernaut who doesn't have his itemisation all over the place.