One big issue with the game is the relationship between nation size and potential city size. A tiny nation simply cannot support a level 5 city -- food density is simply too low. It _might_ be possible to create a large city in a small nation through heavy use of Nature's Bounty, but I seriously doubt the viability of such a path.
It might be possible if minor states were a more significant part of gameplay. The ability to create a minor state out of any conquered city would be a huge plus -- give them a city, no sov, and a number of units, and make them friendly to their liberator. I'd also keep them out of the standard diplo interface, instead requiring an adjacent hero or special purpose unit (say, ambassadors/diplomats?) to interact.
Units near the city state would have a gift button, which could be used to trade a lump sum of gildar for a benefit once every ten turns. Click the gift button and a number of options show up:
-a unit that receives bonus to combat on the majority terrain type of the city state's surroundings with stats based off the purchasing civ's tech accomplishments
-military assistance in an ongoing conflict for the next ten turns
-a single unit of a resource for ten turns based on terrain type (if in hilly terrain iron, if on plains horses, etc)
Only one can be chosen during any ten turn increment.
Units that can access the gift interface can also influence the city state, slowly building a relationship with it. If sufficiently influenced it will make a offer a military alliance and provide either a unit every ten turns or access to a resource. Influence could be checked from either the diplo screen (where it would appear next to the city state's name and the bonus it provides in a list of known city states) or by clicking on the city state. Heroes would generate influence at a rate based on their charisma stat, while special purpose units would generate it at a fixed rate (equivalent to a hero with charisma == 10). Influence can only be generated if the unit is not a member of a stack, and a nation can only have influence generated by a single special purpose unit adjacent to a city state.
Other nations can counter this influence by moving their own heroes or ambassadors adjacent to the state and attempting to influence it themselves. Nations can also use a spy (kingdom) or assassin (empire) to counter special purpose diplomatic units. Spies would have the ability to reduce the amount of influence a nation is generating by a significant amount, while assassins would roll for a kill against the charisma stat of the influencing unit. Assassins can only use this ability against units moving alone. Both spies and assassins would require a dexterity check for a hero to spot them (special purpose diplomacy units roll spot checks as if they have a dexterity of 10), but are visible to one another.
Any unit capable of generating influence is also capable of declaring war against the city state, again through a button. There are multiple war types available for use against city states:
War of Conquest: city state is annexed into the nation if it is occupied, city state auto-requests assistance from all nations with influence in the state. This request can be rejected without any diplo penalties, although some influence is lost.
War of Dominion: can only be declared if the city state is in an alliance, city state auto-requests assistance from the nation with which it is allied. If the nation refuses to intervene, it receives a "You are Weak/Dishonorable" diplo malus against it from other nations. On victory, city state remains independent but transfers alliance to victor.
War of Influence: can only be declared if multiple states have influence over a city state but no single power has it in its sphere. All states with influence are auto-requested to join, can be rejected without diplo penalty but causes a loss of all influence and reduces generation for ten turns after peace deal. Winner gains influence in state, losers are zeroed and cannot gain influence in that city state for twenty turns.
tl;dr:
Ability to create, influence and fight over city states would add a great deal of fun ways to play the game, as well as all sorts of reasons to engage in limited wars. Would also provide a rationale for non-military conflict in the form of influence and spy/assassin activity. Should be coupled with the creation of a casus belli system, more on that later.
