What the Mordekaiser Rework SHOULD Look like. (versus Meddler's goal to make him a dull tank)
One of the options we're looking at at the moment is whether we should play up Morde's tankier side a bit more. Huge guy in a full suit of armor with a passive that generates a shield suggest tough front line dude. The removal of DFG already shifts him away from bursting down a target almost instantly, our thinking is that offering him a more sustained presence might be a good approach as a result. That's very much an idea being explored right now though, not yet a proven direction.
I doubt this will be seen BUT I would like you to consider looking more at mobility than reworking his abilities all together. As it stands, Mordekaiser works well as an AP Bruiser in the top lane save for his lack of mobility. I've found that he can lane similarly to the likes of Lissandra and Cassiopeia in that he can harass as he farms and exploit missteps by the enemy Top Lane. What he LACKS in comparison to such champions, though, is mobility in his kit. As such I only pick him in team compositions that can supplement that lack of mobility (i.e. If I have champions such as Thresh, Lulu, Orianna, Jayce, etc).
I'll also usually only pick him in team comps that have the potential to quickly kill a champion (usually the enemy AD Carry) marked with his ultimate. But back to his mobility issues...
Ideally (at least as a starting point) he probably only needs a Rage/Furor Passive on Mace of Spades that starts as a nice boost of speed then diminishes over a short-ish duration of time.
From this point there are two other abilities in his kit that could be used to provide mobility - Creeping Death and Children of the Grave. These would provide movement speed for Mordekaiser for that duration of the abilities (in the case of Creeping Death, it would only provide Movement speed for Mordekaiser and only if he casts it on himself).
Here's a mock-up of these abilities: Mace of Spades (Q): On next hit, Mordekaiser swings his mace with such force that it echoes out, striking up to 3 additional nearby targets, dealing 80/110/140/170/200 (+100% bonus Attack Damage) (+40% Ability Power) Magic Damage. If the target is alone, 132/181.5/231/280.5/330 (+165% bonus Attack Damage) (+66% Ability Power) magic damage is dealt instead. Mordekaiser also gains 40/50/60/70/80 Movement speed that diminishes over 5 seconds. Creeping Death (W): Unleashes a protective cloud of metal shards to surround an ally for 6 seconds, increasing their Armor and Magic Resistance by 10/15/20/25/30 and dealing 24/38/52/66/80 (+20% Ability Power) Magic Damage per second to nearby enemies. If Mordekaiser casts Creeping Death on himself he gains 3/6/9/12/15% Movement Speed for the duration of the ability. Children of the Grave (R): Steals 24/29/34 (+4% Ability Power)% of target champion's maximum Health (half stolen initially and half over 10 seconds; deals Magic Damage).If the target dies while cursed, their soul is enslaved and will follow Mordekaiser as a ghost for 30 seconds. Mordekaiser gains 20% of their Ability Power and Attack Damage, and the ghost's stats are enhanced. For 5 seconds when Children of the Grave is cast, Mordekaiser gains 50/100/150 Movement Speed and for 5 seconds after that the bonus becomes 25/50/75.
It would also be really great if his abilities had shorter/no cast time.