What's In Between the Sea of Rhun and Southeast Mirkwood? (Late Third Age)
I'm familiar with most of the answers about blank, unknown or underspecified areas of the late Third Age map of Middle-Earth based on hints from Tolkien's notes and more fragmentary drafting (say, Gandalf's explanation to Boromir of why the Fellowship can't just go down to Minhiriath and Enedwaith and come into Gondor from its coastal west) but I don't recall anything describing what's in that big empty rectangle that runs from south of the River Running, west of the Sea of Rhun, east of Mirkwood, and north of Mordor. I don't have any sense of its terrain, or whether there are any people to speak of at all. Gloin says that the kingdom of Dale now has expanded its authority to south and east of Erebor and Esgaroth but it's not clear how far that goes. I guess the answer is arguably "Rhun" since it's in big letters on the classic map but pretty far over to the East, on the other side of the Sea of Rhun.
Did Tolkien ever have anything to say about that blank rectangle?