What Should Mordekaiser's magic type be?
Mordekaiser's new lore and most abilities are centered around necromancy. His old kit was all about metalomancy (that aspect is still visable in his W and passive to an extent). His old lore was all about pain manipulation.
What should he use when he gets VGU?
Necromancy? It's already heavily represented in League and there is not much more you can do with it, without making it feel flat or not overly original. Of course it is possible... But isn't it better to explore more options?
His old thing... Metalomancy. It's original, or at least not featured with any other League champion, and has a lot of potential... Does it fit though? How would he learn metalomancy? Getting it through his resurrection wouldn't explain much - necromancy fits much better in this regard (the learning process could be changed though). It also doesn't fit either the tyrant or the death knight side of his theme.
Pain manipulation? It's interesting, to say the least. Once again - no other champ in league has that (There are sadistic champs who enjoy pain, but none who can controll it). It would fit his identity, could be explained through his resurrection. It also combos extremely well with his warrior side and could potentially make for some great abilities.
Out of the three I have to say I like the third option best... However. What about different magic types - ones he didn't have before?
Lets start with one his ult is most based on: Soul Manipulation. Soul Manipulation is a lot like Pain Manipulation and Necromancy combined. Combined is the imporatant part. As many have pointed out before; Mordekaiser has some serious issues with theme-overload. Making Mordekaiser control both Pain AND use necromancy would add more trouble. Combining two themes and creating a single one with traits from both removes that issue. Soul Manipulation would also fit Mordekaiser's other characteristics, especially that of an Overlord.
Next - A theme much less unique and original, but very versatile and fitting both Mordekaiser's idenity and even moreso his lore. Witchfire / Dark Flames / Corrupted Flames - however you want to call it. Mordekaiser's body was burned on a pyre. Would be pretty poetic if he used a corrupted version of the flames, which were meant to destroy him. It'd also make for some amazing visuals.
Speaking of Magic birthed from his death and resurrection... You know how Morde is made of this black smoke thing? How about using that? The black spilling out, embueing his weapon and used to damage his foes. Could be used for some extremely cool visuals and abilities.
Related to the previous one: Black Mist manipulation. Visually it would work like the previous idea, but the lore implications would be different: Mordekaiser is shown to be above other Shadow Isles undead, who are controlled by the Mist. Controlling the Mist would fit this idea a lot.
cause he don't work no more due to 
need the black mist. we dont know about
but he should need the black mist.
i think having mord center around the black mist would work really well having be able to will and summon the black mist by force of will. my idea for a new passive would be sort of like
where the mist rises up around him instead of tentacles, but also champs he kills spawns Mist.