The problem with this is that MRD has nothing that supports spell casting. It has the lowest MP pool of (i think) all classes, all of its abilities are physical, and no way to regen that MP. So it requires a lot of shoe horning to get the MRD skill set to work with casting spells.

On the other hand GLD already has some basic support for use of magic. It has a higher MP pool, flash as an MP ability it already uses, and the riot combo to regenerate MP. If they were to make a melee/magic DPS hybrid, I would think it's much more likely to be based on the GLD.

However, I don't think that should be the DRK. The big thing about DRK has always been Souleater (as has been stated), the ability to sacrifice life for increased power. And this can function entirely as a physical job. It makes much more sense to base this off MRD (as bloodbath and storm's path can be used to restore life lost from souleater).

On the other hand, I think GLD/ACN+THM could make a very capable RDM (DPS).