If it's a "Cities in Motion"-like UA then it's almost certainly the Sioux.
I see it like that : Cities can be founded anywhere in neutral territory. Cities within 3 tiles range of a fixed city are nomadic ie. can be relocated and therefore cannot grow territory or work tiles outside immediate vicinity. Cities beyond the 3 tiles limit can settle down permanently.
Yeah Eagle I agree. Another fear I have is that the AI can not handle its unique nature and having the civ on a map distorts balance due to its weakness. I think they are hyping this too much and its going to disappoint.
How would Mobile Cities work?
Maybe they want to reflect losing land to the white man? So lets say you build a "city" or a tribe that can move. Maybe it has automatic access to everything within range 3 of that tile. That could be a HUGE bonus early in the game. But then opposing civs could contruct cities anywhere in that range, effectively forcing that tribe to relocate after its key resources fall into the hands of the opposition. Maybe to disrupt expansion, they rely on barbarian conversions to sack settlers in motion. So the UA could be:
UA: Cities start with 3 tile range borders that remains neutral land. Barbarian camps can be captured and used to spawn Tomahawk UU.
UU: Extra movement and flanking bonus. (To outpace and sack opposing settlers)
I actually really like this idea. It would be reminiscent of the Lakota nation from Rise of Nations: Thrones and Patriots. http://riseofnations.wikia.com/wiki/Lakota
They really don't have borders and can use neutral land and are "mobile." Having barbarians raid civs and capture settlers without declaring war would be interesting to say the least.
It wouldn't really confuse a human player, but would work well against the AI. And at the very least, it would give the NA civ player the ability to harass and stall encroachment on their lands.
X-Com Civilization.
or.... the capacities for us to plays as a city states?