Promotions. The promotions are going to be representing a couple of things... once you've discovered domestication, when the tribe encounters new animal resources, they get a promotion with a picture of the animal which opens the possibility to get cavalry units... once you have the necessary tech to get cavalry units.
Promotions will also represent specialists moving with the tribe... think of it like buildings but without the... uh... building. And these people would affect the percentages of spawned units. As the Camp grows, more promotions become available. They're not earned through combat, but are given each time the camp grows one size.
But I figure that no player is going to be fully nomadic for more than 50 turns... By that time, they'll probably have captured a barb city or some other player's city by then and the camps will be more for providing additional units.