Short term fix for Mordekaiser (not accounting bugs
A good change for 
would be the approach riot did to 
for short term fix :
-Making shield activate when its fully filled seems a good approach
-Make it way harder to activate. (Like 10 E hitswich means it would fill up 10 % of his shield)
-Therefor make scale with 50% of his Hp
-Make shield last for 5 seconds up to 15 seconds max when in combat. (champions big monster)
-Give the ability to refill his passive past that point
-If shield is expired it has a 15 second cool down before it can be filled again.
-Make q w e and r become stronger (like raid boss darius stronger)
- Powered up Q: Mace of Spades = shot stun on third (0,5 sec)
-Powered up W: Harvester of Sorrow = Harder heal ad bigger movement speed
-Powered up E: Siphon of Destruction= pull them slightly towards you when hit with the skill (smaller than threshs e)
-Powered up R: Children of the Grave = grievous wounds effect
That way you would be able to make stronger, more viable and give the enemy a window of opportunity to deal dmg to .
would be also more lore accurate in reason of being the all mighty warlord wich cant be beaten alone.
You can even give a 
treatment where you make all of abilitys skills scale with ad but deal magic dmg.
I guess that might need some nerfs for these kinds of buffs.
Please note that those things might not account other weaknesses of but it would be hopefully the first step into the right direction.
Also do not forget that the Runes and mastery will be gone this year which means that wont have stormraiders or any other helpfull ability and Bloodmoon hunt (upcoming keystone Rune) will end as soon as you are in combat which might spell trouble for you beloved ironman.
If you took yourself the time and read the whole thing thank you for you patience.
Let me know what you think pls. [slayer-pantheon-thumbs]