The “Great Kingdom” was a kingdom said to be enormous, prosperous, and powerful. It was likely an enemy of the Twenty Kingdoms (which is now the World Government), as implied by Professor Clover, and was seemingly utterly destroyed by them over eight hundred years ago at the end of the Void Century.
In fact, it seems their defeat allowed the Ancient Alliance to truly become the World Government. Furthermore, it is the real reason the World Government has banned any Poneglyph deciphering and research as the mere “existence and idea of this kingdom presents a threat to the World Government”.
My speculation is that there were 7 families of the D Clan that ruled within the Ancient Kingdom. We know 6 of those families:
7 Rooms in Mary Geoise. Harboring the 7 family heirlooms
All rooms were frozen as to preserve a body. 7 bodies = 7 family heads and their heirlooms. No other reason to freeze the rooms if you are not going to preserve organic tissue.
World Government and their 7 Warlords
A possible attempt to mimic the miltary structure of the Ancient Kingdom. Teach and Law both joined this organization unbeknownst to the World Government; a possible foreshadowing by Oda. Ace was also formally invited but declined. That’s already 50% of the known D. clan members that either joined or were offered the position…
*Theory by OrangeAdmiral