. . . Camps develop into City States, which was already implemented in Civ IV in the past, is an okay idea on its own - these people may not be full Civs due to various reasons, but they were not "barbarians" either.
So while we're on the subject of City States vis-a-vis 'developments' in game, how about Upgrading them a bit while we are talking about potential developments from Tribal Camps?
Right now, City States are extremely Passive. They have no active diplomacy, are merely recipients of Envoys and influence. They initiate only 'generic' trade, no specific Resource trading and haggling. Their armies, although large for the early game, are completely defensive - they will go after nearby Barbarians, but only attack other Civs if Suzereigned, never initiate conflict. They do no research, develop no Religion (they are utterly passive in that aspect of the game, too) and appear to have no cultural interactions at all.
Using the new system of Upgrades from Camps to City States, if we aren't for the foreseeable future going to get Upgrades from City State to Not-In-This-Particular-Game-Yet Civilizations, how about 'lesser upgrades' of City States into, say, Minor Powers?
There are a number of potential actions/activities these could have:
1. They have diplomatic interactions - maybe a generic Audience Room instead of a fully-animated Leader - so that you can demand/request/beg for specific actions from them - intervention, for example, against a specific Hostile Camp nearby, or a Trade Agreement for a Resource, or to establish a Trading Post early in their City. They might request things specifically from you: trade us some of X Resource, pay us X Gold.
2. They should have their own Technologies/Civics. They may not have separate Research (that could get very complex very fast) but a simple 'osmosis' mechanic could allow them to acquire technologies from nearby Civ or Civs they trade with (similar to how they acquire the ability to Upgrade their units now, but expanded). That might also allow you to acquire Technologies from them (and an expansion of Peter's UA)
3. If we can now 'hire' Tribe/Barb units, why not be able to hire individual units from City States instead of, as soon as the Camp becomes a City State, be limited to an All Or Nothing 'hired army' mechanic? Add a collection of City State Specific UUs for militaristic City States, and we're on our way to a real Mercenary system in the game, which is also Long Overdue.
4. If you can Suzereign a City State, and gain Cultural Influence over them, and spread your religion to them, why can't you absorb them? In other words, at some point why doesn't the City State decide to join your civ?
5. And, why don't City States cooperate with each other? Why not diplomatic relations among City States, so that attacking one of them may involve you in More War Than You Wanted with that state and several of its neighboring City States? Or find that the City State that you've Suzereigned is going to war with another City State and is requesting your help - and that other City State is part of the 'sphere of influence' of another Civ and you may get dragged into a Major War you don't really want (right now, anyway). It is probably too much to have a little graphic Archduke getting assassinated in a City State, but the idea and concept should be there . . .