There are some balance mods, and some "reimagined nation" mods, and an EA Machaka nation, and I did a small mod that added an Anansi giant spider pretender for Machaka... Other than that, it's probably because the nations all have their own themes and stories, and they are very strong. It's like, well, sacrilege... The Code of Illwinter is Law, and we are Not Worthy!
Oh, and the hero mods sometimes add or modify the national heroes, sometimes adding additional descriptions or links to other nations.