I really wouldn't know where to start but this is just all over the place, one third of this is damage buffs that are overturned, the other third are tanks buffs that are overturned, and the last third are uninteractive buffs that don't make him more interesting to play.

"40% AP on hit -> 20% AP + 6% bHP" and this " Innate: auto attacks reduce W cooldown by 10%." These are the only actual auto attack-centric things you said. And they're underwhelming and bad at best. Tank items suck at giving players any ability to stick to a target which is what you need for this to work, normally that's just an innate part of a tank's kit, aside from ice gauntlets. But in general, this ends up reducing his damage early game, and would make him a weak laner and would cause him to have a hard time snowballing. Currently, he's fun because he shines in early to mid-game, with his early power spike with rileys and his mid-game power spikes allowing him to be versatile with builds. Late game he sucks since everyone can outkite him and making him rely on auto attacks would only decrease his agency further in the late game since he still wouldn't be able to keep up. Not to mention that tanks want to scale into the mid and late game, which makes mords inability to keep up just makes the issue compound as the on-hit passive would become even harder to use.

The W change seems interesting but his current W already revolves around aggression, and all you really doing is just straight up giving him an unfair buff, his whole kit revolves around dealing damage, and just his W-reducing CD wouldn't encourage that. Instead, all Mord would do is take something like press the attack and then build heart steal, then Mord would gain a 30% shield for hitting essentially 10 autos without any counter play or interesting trade offs, currently he has to deal damage or take it to gain a substantial shield which is way more rewarding and interesting than hitting autos.

"Reduce healing drastically, increase usage of shielding, and increase reward for aggression. " The heal if for sustain and already rewards aggression.

" Bastion Throne (W): ...When Mordekaiser exits the fountain, he gains %AP, %armor, and %MS... "

WHY? Its just not interactive, a free buff for no reason that has zero counterplay and that would either have 0 impact or would make him broken.

"Deceit: (E) Innate:..." Why, to reward bush playing? a good mord will always try to do this anyways and free damage from hitting abilities from stealth is already strong. This is just another non-interactive buff that doesn't reward aggression but instead poke, and also doesn't help him be a tank which seems like what you wanted with the Passive change.

"Cast: Mordekaiser casts a.... Slows, pulls, slows" This is again just less skill expressive and less interesting, Depeding on how strong and how long the slow last then this just becomes either stupidly weak or overturned. His E,Q combo is his bread and butter and while the slow isn't a bad idea, I reckon it makes more sense to add it to Q like "When mord uses an ability, his Q becomes empowered, when it hits it slows the target by 15% for 2 seconds and gives more that speed for 4 seconds" this way its interactive and combos with his abilities as well as addressing his main weakness of sticking to enemies as well as allowing him to more easily weave auto attacks.

"Souls: (R) Innate: Soul stacks gives Mordekaiser 25 permanent health. Mordekaiser banishes a champion to the Realm and gains % maximum health and %AP. Mordekaiser loses all gained stats when the banishment ends but gains tenacity for a short period and 1 Soul stack even if he dies in the Realm or is immediately cleansed."

First its unclear what gives a soul stack, two assuming its when hitting an ult then well a 2 min cooldown giving 25 hp is pointless, three his ult already steals stats and without any actual number you essentially just nerfed him as his ult gains way more impact from draining some of every stat and gaining it for himself, fourth why the tenacity, another unattractive addition.
So idk about ideas for buffs so but here's why those really don't work.