Interesting idea. I agree that Urban Native American and Nomad Native American should be in different groups. The restriction for Nomads would be the one already said here, less tech but combat bonus against higher tech and may also get techs from killing units; perhaps also giving settlers extra movement or something. Then, Urban Native Americans shouldn't get less tech; just probably make oceans impassable, and horses unabled until meeting another culture group. In exchange, bonus for wonder production in the capital; and extra tourism. (Which is going to be interesting, Urban NA would be influencing the local cultures, but all that tourism is still in no effect until meeting other culture groups) As for Colonial... Somehow I think ideological pressure, or something related; can't think of something specific.