As someone who tried Mordekaiser on the PBE and tested out ways, I must say:

His solo lane has gotten better and his duo is still weak if he goes AD.

Reason:

I went Mordekaiser and used critical items that give both AS, MS, or AD. I also got runes that would help with it, by getting MS bonus runes, health regen runes, and MR and Armor to be safe. In his masteries, I went from the tanky Mordekaiser to the more AD style while also adding additional MS (the sub total was 6% bonus) and now he has almost 345 when he starts.

The sad thing is, I tested it out on a Yasuo, whom Mordekaiser completely negates. Both scale off shields and stuff. I even had a thing going with me vs Yi and when I went all in, I won. This is Master Yi at max level and full critical build.

His Q is great for pushing still. But his AA is way better.

His W is great for junglers to gank, and they feel safe to do so.

His E is still the same.

His R holds so much power.

The change isn't fantastic but I figured out that this can work.

Now, in his duo lane.

His Q cannot reach the opponent half the time. He gets over crowded and his passive doesn't work properly. His E works the same. His W is active more, but it doesn't deal much or heal a lot when building AD. His R is his only real power. His is also vulnerable to ganks and with his MS and his big model size he is a easy target. It will be hard for him to get back to turret.

His duo should be buffed if that's what they're going for.

(The only reason I'm posting this here is because the PBE boards are very silent right now and little to no REDs are commenting there.)