Out of the outlier cultures on the fringes of the maps, Dorwinion does seem to me to be the most likely to produce Hero characters. And existing material for TOR already gives us some hints of what a Dorwinion sourcebook might look like:
I still think there is a good chance of East-elves living in the woodland east of Dorwinion. And there might well be a hall belonging to Dwarves of the East in the hills to the south.Dorwinion
Dorwinion lies hundreds of miles downriver from Esgaroth, surrounded by the River Running bordering the inland Sea of Rhûn. It is a land of merchant princes and princesses, who trade far and wide; with the Elves in the West, Esgaroth in the North and even as far as Gondor in the South.
Dorwinion is particularly known for its vineyards, from which the finest of wines are made, much sought after in the courts of Men – and Elves. To the Northmen, the folk of Dorwinion seem exotic, with dusky complexions, deep, dark eyes, and clothes of brightly patterned cloth; ’tis no wonder that King Bard was so enchanted by his queen, Una.
The section in Erebor on the Nether Marches could apply equally well to East Rhovanion. This was the home of the ancestors of the Éothéod before they were driven to the Vales of Anduin. As in the Nether Marches, some of their folk would remain:
And there would need to be coverage of the Sea of Rhûn and some of the lands of Rhûn. From the booklet for Journeys & Maps:The Nether Marches have ever been a frontier for more civilised lands; just as they now mark the south-eastern limits of Bard’s kingdom, over a thousand years ago they formed the northern borders of the King of Wilderland’s domain. While his kingdom was all but destroyed in the Great Plague of 1635 and, later, finished by armies from the East, some of his folk yet remain. They follow their herds of cattle across the plains or make their homes in small fishing villages along the Running River, but they owe fealty to no king.
Past the Iron Hills and the Redwater, and extending far beyond the edge of most maps, lies a vast domain known in the West only as Rhûn. From the great inland sea to forests, mountains and plains, this expansive and diverse terrain will present a wide range of challenges for travellers. Wild oxen and other unfamiliar beasts roam these remote lands. But perhaps even more dangerous are the Easterlings, the Men of Darkness who dwell here. Sworn enemies of the Free Peoples, the Easterlings are fierce warriors and allies of Sauron.