A few I can still remember (it's been a while!):
Passives:
-Every kill/assist enslaves the enemy's ghost, forcing them to serve Mordekaiser for a duration and assist him in attacking his target. He could have up to all 5
-His basic attack speed was exceptionally slow, but auto-attacking enemies would grant a stack that significantly increased his attack speed. The metaphor here was his mace was a living weapon, and that attacking enemies allowed it to create an ethereal chain and become more of a flail than just a mace, thereby giving him that attack speed and additional range
Q:
-Early testing was done around what it might be like to have contextual changes to his Q shape. We messed around with things like: when you press Q, he starts marching forward in a straight line, then when you re-press Q, the location of your mouse relative to his facing would give a different swing. Something like:
Forward - if the mouse is in front of him, he slams his hammer down forward.
Either side - if the mouse is to his left or his right, he sweeps it from front to back in that respective direction, knocking things forward and a bit sideways
Behind - He either heaves his mace backwards, dealing significant damage behind him, or he just does a larger swing that deals damage behind him as the mace goes from behind to in front. Longer cast time than the forward Q, higher damage
W:
-This was the last ability to get locked, so there were a lot, but they were all focused around defensive outputs. We tried things like: he becomes his wraith form and is untargetable but can't attack. He gains armor/MR based on nearby enemies and becomes immune to slows.
E:
-My favorite ability we tested here was: he shot a circular missile forward that moved very slowly and opened a rift to the death realm. It would deal damage to enemies it passed over and put them into the death realm temporarily while they were in it, making them unable to interact with people outside (just like the R rules). Morde could also walk with it to be inside the death realm as long as he moved with it, making him untargetable while he approached enemies
R:
-This was basically always the death realm from the beginning of the rework, but we tried a lot of versions. The first version we did was:-Morde targets an enemy and a circle appears around them and Morde. After X seconds, everyone inside those circles is pulled into the death realm and all of those folks inside can fight.
-That ended up being too difficult to actually get into a lot of times, so we changed it to: after X seconds, clickable objects (like Thresh lanterns) are left where Morde and his target were when they went into the death realm, and anyone can click on those to join the fight in the death realm.
-All of those multi-person versions just became a chaotic mess that wasn't very fun to participate in, so we gravitated towards the 1v1. Then we tried a lot of different cast paradigms, like shooting a longer-range skillshot (like Urgot R) that would take the first enemy hit, to a version where Morde's spirit exits his armor and slowly floats towards the target and the first enemy his spirit touches goes into the spirit realm, etc.