All numbers aren't final and are just there to give a general idea.
The goal of this rework was to align Mordekaiser's skills with their names and to develop his character as a tanky, armored man who's going to steal everything from you and torture you.
-Health * Level removed
+Maximum health scaling.
This change is designed to give an incentive to build health. This change means that early on it's a nerf but end-game should you decide to build health you'll gain a higher and higher shield than compared to the current Mordekaiser.
-Isolation damage removed
This change is meant to turn Mordekaiser into an AOE bruiser. The isolated damage is removed because I feel that it's unneeded now that there's innate CDR and it had the problem of "Why would I want to have isolation damage if I'm tanky enough to survive 1v1s anyway but also I would have a 160% AP ratio if I used it in a teamfight. The spell vamp I think is needed as all health-using champions have a way to regain health without their ultimate, and it's going to be the only way for him to develop his shield pre-teamfight, if he ever doesn't get WotA, if he decides to build tankier first (Thought I have no idea why anyone would.).
-Removed armor and MR buffs on ally
+Ghost gains buffs
+Slow + Root + Stun added
It really does become Creeping Death. The Slow -> Root -> Stun allows for CC if Mordekaiser stays close enough, and allows for the rest of Mordekaiser's combo to come off. Slowly shredding their resistances, dealing damage and gaining a shield. The ghost now gains buffs as it's an essential part of his gameplay and allows for strategic positioning to make sure the enemy gets CC'd even without the MS buff.
The MS is removed because I don't think it matches Mordekaiser's character. I see him as a tanky bruiser that walks in and slowly and surely takes everything away from you. And why would it be creeping if he's running towards you?
The armor and MR buffs is removed for the same reason Lulu's E shield was removed on minions, it would make minion's hard to kill, allowing for free damage that's hard to get rid of.
+Ghosts get resistances
This change really had to be changed, it's the dullest skill, basically being an AOE damaging cone with no extra effects. How fun. This change was to make the skill 'actually' siphon something. This will turn Mordekaiser and his ghost more tankier, so even with the MS from his W removed he can still survive the walk inside.
No changes I think this skill is alright.
These are just my own thoughts that I posted here for fun. If you do give criticism please make it constructive and thank you for reading. I realize that I changed Mordekaiser a lot.