Known locations of Godstones in ME according to Pymel the Meticulous (IIRC): "Where dwells the White, the Black and the Grey one - and too the Fiery Fiend - there they stand abroding" (that's from memory).
I never quite understood that reference. But I am thinking it refers to Minas Tirith (white), Minas Morgul (black), Lindon (grey), and Moria (fiery fiend). In any case, there definitely is one in my p-Moria. I don't think this campaign will encounter it though.
I also went with white = Saruman/Orthanc, black = Sauron/tower, grey = Grey Havens - but I like your ideas as well.
Both Minas Tirith and Minas Morgul were Neumanorian weren't they? If association Numanorian = Earthmasters (posiably powerful enough) could probably work best - but still wouldn't explain Moria.
Orthanac, Black Tower and Moria though could just be random places of power that the Earthmasters used before anyone else got to Middle Earth.
I'm thinking Minas Tirith and Minas Morgul are geographically too close for reasonable earthmaster sites. Orthanac, Black Tower, Moria and the Grey Havens though are nicely spread out though.
Okay, this is just too good to miss.
I like the idea of associating Maiar with the godstones.
Keeping that in mind, it is IMO fairly clear that Saruman/Orthanc=white and Balrog/Moria=The Fiery One.
Fairly clear is also that Black=Sauron/tower. However, I'd like to suggest the possibility of the tower not being Barad-dur (the Black tower) but Dol Guldur
("Hill of Sorcery", Sauron's fortress in Mirkwood). Wikipedia claims "Tolkien suggests that Sauron settled on Dol Guldur as the focus for his rise during the period before the War of the Ring in part so that he could search for the One Ring in the Gladden Fields just up the river." referencing "See for example, The Silmarillion, "Of the Rings of Power and the Third Age", p. 302.".
The grey is more unclear than the others. Gandalf the Grey
didn't have a permanent residence and traveled a lot, but indeed had connections with Cirdan at Grey Havens. Pymel the Meticulous might of course not have known this and referenced Grey Havens as Gandalf's residence. So I'd accept Gandalf/Grey Havens=grey.
All of the above are Maiar and are IMO consistent with being references to the time in the Third Age, before the War of the Rings.
Men of Numenorian descent i.e. the Dunedain, who lived in Arnor and Gondor (Minas Tirith), are not in my opinion strong enough to be considered Earthmasters. Numenorians, maybe but unlikely, unless you go back in time when the Island of Numenor was still on the face of the Middle-Earth and the people not corrupt (the White tree standing).
There are more possibilities, if you go waaay back in time, even to the First Age, and consider for example Gondolin as the white spot, Morgoth as the black one, Gothmog (lord of the balrogs) or Glaurung (the father of the dragons) as the fiery one.. Dunno about the grey one in this case though. However, if the Gargun are somehow related to Sauron's orcs, then this kind of an interpretation is not probable.
One last thing. IMO it doesn't matter if the places are scattered or not. What matters more is the story! If you've got a plausible reason for both Barad-Dur in Mordor and Minas Tirith in Gondor, go for it. After all, they're even places of opposite alignments!
Edit. Post #100 btw, woohoo!