I personally think that Children of the Grave is one of the most unique mechanics that Mordekaiser has in his kit, and I don't think it should fundamentally be changed in the sense of "when the marked champion dies, use his corpse to kill the enemy team". However, I do feel that there is some stuff that could be updated a little.
I personally feel like the heal on his ultimate makes for some really weird use cases with his ultimate, where he wants to use it early in a teamfight on someone with full health to get the healing off of it, but also wants to use it at the end of someone's life to get the ghost. The heal also doesn't feel too satisfying at all because it is over such a long period of time (10 seconds), and is only practical in 1v1 situations where you know that you will not survive if you don't use the ultimate. I literally 1v1'd a fed enemy Rengar by using the ult on him while I was at less than half HP and he was at full. He was so fed that after I killed him, the enemy Fizz came in to try and clean up and the Rengar Ghost nearly killed him while I was in Zhonya's.
On the other hand, it can be frustrating to ult someone at low health and they sustain enough to just barely outlast the heal. Grievous Wounds might be cool on it.
My suggestion: remove the heal, and replace it with Grievous Wounds. This makes its use case of being a finisher a lot clearer while removing a lot of the hidden power it has in the weirdly long duration heal.