At the start of the game, it makes sense that most of your gold income comes from fighting monsters, like a plain old adventure game. Right now, by about turn 100, things going well, you are probably making a profit of 40-50 or so gildar per turn from your cities and the creature economy becomes less important. However, it's not insignificant. I don't think that's appropriate – as you grow from a band of nomads to a proper kingdom, the gameplay should change accordingly and become more complex. It doesn't make sense that income from fighting monsters should even be of the same magnitude that your whole kingdom makes per turn. This is supposed to be a grand strategy game, not Diablo II.

After the economic system in the game gets more fleshed out and complex and more mechanics are added, such as taxes, the creature economy should become even less important; you'd be more interested in using your troops for wars rather than cleaning up spiders from the local woods. Since you are playing as a ruler of a kingdom and not a lone adventuring party, you should be a quest giver, not just a quest taker. I think this could be solved by adding a bounty system similar to Majesty, which fits in well with the fantasy theme and would give those wandering NPCs something to do, as well as get some money flowing through the game economy. For this to work, the NPC heroes would need to be given a small party to start with. If an army of creatures are bothering you, you put a bounty on it and eventually an idle NPC hero will come along and try and take it out. If successful, they get the creature gold plus the bounty you placed on it, which would come out of your reserves. The base bounty you should offer would be based on the CR of the army; if you put a larger price on it's head, it would likely get taken out sooner because more heroes would be interested in going after it and vice versa. Now your lands can be kept clear and your troops free for more interesting battles.

The battles should allow heroes to level up too, though maybe not as quickly as civ heroes, while correspondingly increasing their hire value. This means that as the game progresses you would be able to hire some strong and interestingly designed heroes, because the levelling decisions are made by an AI personality and not the player. At an increased cost of course. If you pay more, you could perhaps hire their party as well. Maybe some NPCs will be set not go after bounties, so civs that can't afford the experienced ones can still hire low level heroes.

When I refer to a “player” here, I refer to AI players as well, not just the human. They too will be able to set bounties and take advantage of the mechanics.

To complete the circle, the NPCs should use the money they earn to upgrade themselves and their party with new equipment purchased from cities of nearby players (the players could get the money directly, or they may earn just part of it from an adjustable sales tax rate, whichever works). Other things could be for sale as well. When item crafting is introduced, players could put these up for sale. NPC heroes may want to purchase whole new units for their party. And so on. I have seen other suggestions for economic options to be available in cities. They would work well in this model, as they give the NPCs more stuff to spend money on in the civ cities, returning cash into the player's coffers. This will provide interesting new building options for the players... you could try to build a luxurious recreational city, with bath houses and bazaars, a fantasy Vegas, to attract free spending NPCs, while the AI does the same.

Collecting bounties need not be limited to the NPCs either. Players could sign some kind of bounty related agreement with each other, which means that if one of the players sets a bounty on an army, the other player could go kill it and earn the bounty on top of the base gold. The agreement system will prevent players from having to pay other players that they consider enemies or competitors, as they wouldn't want to give them their money. This raises the possibility that you or an AI could play a mercenary type role for other civs, providing an alternative mode of play for a time when steam rolling every opponent in sight is no longer always the shortest path to victory.

The bounty system could be used for more than just creatures. Perhaps you could place a bounty on an enemy city itself (though it would have to be a huge one). This would be a little more complicated as there'd have to be a limit on the number of parties that can target a city over a given period of time to save it from getting overwhelmed and there are diplomatic implications if actual players, human or AI, want to accept the contract. But I think this would make the mercenary side of things a lot more exciting. When more economic and builder stuff is added to the game, you could perhaps play the game as a builder, improving your economy and using only a small army. You'd still be able to target enemy cities and generally harass enemy players by using the money you earn. Again, it provides an alternative path to victory than endless battling.

There are many other things that the bounty system can be used for too – as more mechanics are added to the game some of them would allow the bounty system to be extended nicely. This sort of thing will add more interaction to the game world and produce an economic cycle. My point here is that such a system would supplement the questing system, in that quests (or missions rather), will get dynamically created during the game based on the needs and goals of the human and AI players. They would be directed and they would make sense, unlike predefined quests that aren't really connected to the game in hand. While predefined quests can add some nice distractions and please the adventuring players, I think they fit into “adventure” games where a small band of players wander around maps. In a 4X game, you're actually playing the kingdoms that are the reason those quests are there in the first place, so it makes sense that you would be creating these quests for computer controlled “adventurers” to carry out, while you concentrate on empire building and grand strategy.