I support it; it'd definitely be a buff and also bring Mordekaiser up in both play and recognition. As an older Champion Mordekaiser lacks the kind of interaction/unique identity that most new Champs have (he's incredibly similar to
; Spellvamp and offtank-AP itemization, health costs, teamfight-winning ulti, low CDR skills and little/no mobility or cc). The idea for this both gives Morde players some versatility in what they want to use their ult for (ult tanks for the cc disruption, ADC's for the AA damage, APC's for the spell damage, etc...) - as well as giving more skilled players a challenge when using him. This would be like a hyperintense version of Azir's soldiers in practice I think (good god that scares me).
Granted, grabbing the enemy...Syndra, and proceeding to AP-burst a whole team may be perceived as too much - even when you consider the amount of skill it'd take to pull it off. Similarly; you'd end up with Morde's building full tank and being unkillable unless focused with the express role of ulting the most useful member of the enemy team, so that even if Morde just stands still and the player uses the ghost to attack, you can't kill him fast enough.
Still, it's a really awesome idea. All ideas have flaws, but some (like this) are notably greater than others.