The View From Atop Orthanc
''
And so from this lofty vantage point, marking a key milestone upon the grand road we walk upon, we can now look out across the world of Middle-earth and see glimpses of what lies yet before us:

The rolling pastures of Rohan stretch out far to the east until they encounter the mighty Anduin, the armies of Gondor are gathering in the south to defend the walls of Minas Tirith, ''

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so i guess the next expansion is pretty much Rohan