1) Maximus and Tiro from the second game fought on the side of the Knights, but they weren't humans. Yes, in the second game, the Knight faction represents Enroth, not Erathia, but by the time of the third game, they have the same ruler. So, Goblins and Ogres (Tiro is an Ogre, as far as I can tell) have some chance.
2) Beastmen, the inhabitants of Tatalia, were slaves of the Erathians for a long time. And in "Chronicles of Heroes," if I remember correctly, Tarnum is one of the leaders of the rebellious Beastmen.
3) Although Gelu is a hero of the Rampart in terms of gameplay, he is actually a member of a faction working for Erathia. And, therefore, all his Elves and Dwarves work for Erathia.
4) A Djinn-Knight is original, even too much so. Besides, there is one such hero among the Barbarians.
5) The same goes for Troglodytes, Ifrits, Krigans, Elementals, and Minotaurs.

But there is another interesting point: among the Necromancers, there are many Troglodyte heroes, while among the Elves, there isn't a single Centaur hero, and among the Barbarians, there isn't a single Orc hero (up to the fourth part). And yet, the latter have a Djinn hero!