That the "Great Kingdom" was destroyed by the "20 Kingdoms" (now the WG) seems to be a somewhat acknowledged fact. (I'm basing this on the Wiki-page of the "Great Kingdom")
Many might not remember, but Prof. Clover voiced the theorie that, knowing their defeat was imminent, the GK created the ponegyphs to make sure that certain information could not be destroyed. He couldn't say what he thought the kingdoms name was, since he was shot before being able to finish his sentance, BUT he did HAVE a kingdom in mind which makes me belive it still exisis in some form.
We know now that the Kozuki Family from Wano created the ponegyphs. And we also know that the Wano Country is NOT affiliated with the WG.
So my theorie is that Wano Country might be the former "Great Kingdom". What do you guys think?