It all depends on if you want to put agriculture and animal husbandry in. The other way, units would be roughly the stone age ones (you have the choice), as well as the techs and everything else.
So I would say to introduce settles the time you introduces agriculture OR breeding. But this could be done by the player itself. For example, the player would always have the possibility to move his camp, but it would become totally useless with animal husbandery + agriculture.
I mean, the player would have to NEED to move his cities. I don't like the idea to add this feature only "for the style". One want to make him feel the necessity, one want him to feel like in those ages.
So he would have the possibility to move his cities: because of drying up ressources. I think they are more easy to do that migrating animals.
So the system would be composed of drying up ressources and moving cities. The cities would be movable at any time.
Ideally, for a role playing game, the player would have only one camp to move. But we are in "Civilization", so it would have to work the way several camps/cities could be created, and kept small. (around 5 people max) Use the happiness limit here. (a little more severe than normal Civ4)
The ressources would have to deplete quicker than their growth, at least. So that they would truly decrease with time when harvested. Now I don't know how many times quicker, maybe 2, maybe 3, depending on how we need in order to make the player move his city even if it is only 5 people size. (considering he could only change their squares and keep the same city location)
The moving city would act the same way a standard city, except that production would not be affected by buildings.
Buildings could be built, but they would remain in the same location (not move with the city) = creation of a small camp icon on the map.
At the scale of Civ, a city would move all the time. So maybe ressources would have to be made the way they deplete pretty fast (some turns in the best case).
Caves could be found while moving, giving a high defensive bonus against animals and hapiness bonuses. There would be a very low chance to find a cave in a plain or grassland, a much higher one to find a cave in a hill, and a high chance to find one near a mountain. Caves would be ideal locations to build buildings, that could be re-used for several passages.
Now, the camp reproduction. Of course, creating several camps would be useless for Science, as camps in those ages are not federalized. They are separated entities that evolves on their own. So why to create other camps? In a first time, it would be usefull to fight against the beasts all over the map. I'm even thinking by replacing usual units by those camps.
Camps would be usefull to give a Science boost as well. But any one of two meeting camps could decide to fight for the ressources/surviving. Ideally, a unit could not be separated from his camp. Maybe a system of equipment instead of bare units could be created. Maybe the cities/camps would be units with several lives (=number of citizens), losing any when a battle is lost.
The goal of the game would be:
-Surviving.
-reach the better ressources in order to progress in Science.
Victory Conditions:
- Science: Settle down for 20 turns. (with animal husbandry and agriculture + enough strenght to survive animal and other tribes)
- Production: Construction of the first culture achievement. (example: 1 Stonedge, or 20 obelisks over the land)
- Diplomatic: federate at least two cities. (first nation)
Etc... ??
Those are only random thoughts and examples, feel free to keep those you like.