You forget about Ullanor. I speculate that a grand ork like Urrlak Uruk or the Beast might be the end result if an ork empire can settle and gain a modicum of stability with an overlord capable of controlling his fractitous underlings.
If you look at it, so many waaghs are warbosses looking to carve out a name for themselves and get established. Most gets toppled and killed before they can progress to the next stage such as the ork empire of charadon.
Ghazhkull thraka seems well on the way to ascend to the grand ork stage, having an unusual level of control and direction on his growing empire, thanks to his psyker abilities to command through the warp. As well not falling into the trap of trying to do everything himself, but delegating out battles to capable bosses. That to me, shows what the ork mind is capable of once rising to the task.