Mordekaiser needs a niche before he can be reworked again
This is, I think, the most jarring thing about Mordekaiser's rework in a nutshell. Among the Juggernauts, Mordekaiser just doesn't seem to fit in. He has the worst base stats of his group, he's the least mobile, and his play-style is the least satisfying (Skarner is a close second). Worse, he seems to have a very confused build path with hybrid ratios and AD items offering little in actual optimizing for himself.
For comparison, here's what I think the other Juggernauts excel at; why you would want to pick them.

- Janitor; he's built around completely decimating anyone near him and having ridiculous clean-up power if he can activate his passive.

- Anti-Carry; his ultimate's passive gives him good comeback potential if he can get close enough to use it.

- Kidnapper; he has the high moment of being able to steal a high priority target and run them into his team.

- Monstrous damage/set-up; Has the potential to go completely HAM on the enemy team if she can get her tentacles out or land a good ult. Probably the best of them to pick if you want to wombo combo because of the damage she can output in a very short period of time.
Mordekaiser doesn't really have any solid identity in comparison; his passive as it stands gives him artificial tankiness, but the fast decay rate makes him want to emphasize on being able to capitalize on short bursts of shields (encourages damage). He's also lost the identity of being an AoE damage pump like he once was; in addition to his new Q being single-target, every other juggernaut has an AoE threat, usually centered around themselves. He's in a weird mix between an attempt at sustained damage with a lot of his old burst gameplay still being apparent.
I think the biggest issue here is the disconnect between theme and gameplay; Mord doesn't have any level of satisfaction to how he builds, because he can't really take advantage of it outside of just sponging more damage. Darius's damage ramps up; Mordekaiser's goes up and then drops like a rock when his Q is on cooldown. His E is a really weird pseudo-defensive move now; the damage itself is okay but nothing worth looking at twice, and the % bonus of shield is where the value in it lies. His W "promotes" team play by refusing him the option to have the damage when isolated, and his ultimate is probably the only thing that really defines him properly -- the power to drain tank fights and get a noticeable spike in power if he gets a ghost. I feel like this isn't the greatest thing; Mordekaiser's DPS is entirely based around whether he can gain a ghost, and he still retains his 100-0 burst playstyle otherwise. I think Riot should focus him more into high sustained damage and immensely superior durability, as his burst combo consistently proves both unfair and unfun.
If Riot ever planned another rework for him in the same vein, I would like Mordekaiser to function as a super-lifestealer; where he'll rob not just your health, but your armor, your MR, your damage. In return, you could play with his kit; give his E a charge that offers up range, maybe leaves behind an AoE that saps you while you fight him on it, make his W steal defenses from every champion who stands in it, and so on. Somewhat like Trundle, but in a less binary package and tied to the theme of a super-juggernaut who completely refuses to die. Fighting him for an extended period of time should be dangerous; Mordekaiser will make you squishier while becoming more and more durable himself. I don't think his dragon ghost is a well-thought mechanic; it's good siege but it's incredibly unreliable and the ghost itself has wonky controls. If this is a factor that takes up some of his power budget, I'd rather see it doled out to the rest of his kit.
Just a thought. I'm no Malicious Metal but the idea's been appealing to me for long enough I thought I'd write it.
's main damage is in his abilities and those are centered around his weapons.
's rework purposefully made it so there was an ability that interacted with his axe.