White Knight Chronicles had that sort of system in place for certain missions, it was insanely hard right when it released. I wouldn't mind having a Survival Instance line, but I'd sooner see something "Sphere 211" in FFXIV than waves of mobs. (Note that the waves in WKC appeared in the distance on the map before they were targetable. FFXIV could do the same.)

I don't want it to turn hardcore like Coil though.