If DRK would grow off of any current class, it should be MRD, not GLD.
The advent of AF weapons will have to incorporate a system by which when a Job-only weapon is equipped, it will also equip the Soul simultaneously (hence the recent change to make equipping job souls a server-side function).
This will allow new Jobs to have weapons separate from their root classes.
So yes, you could have MRD > DRK (Greatsword)
other possibilities:
GLD >
Red Mage (Rapier)
LNC >
Samurai (Great Katana)
PGL >
Dancer (Fans)
ARC >
Ranger (Crossbow)
CNJ >
Geomancer (Bell)
THM >
Oracle (Cane)
I've said this many times, and I'll repeat it here: We only need a ~5 more classes to have every base style of play covered (see my sig). After that, all growth and expansion should be job-focused.