Well, the trade system is supposed to be another source of gold, savyy? Also, people it would be something that require your total investment, meaning you either was a peaceful nation, or a small nation thriving for resources and then begin conquering.
Man, that's hard. I'll edit if I think about anything that might work.
Edit: Brainstorming... The gold won by territory is by any means taxes from the people who live there. And that's obvious not the other source of gold. In that age, people were in Feudal Ages/Beginning of the Renaissance Age. Only on the very end we have colonization, so it is not a very viable option. At that time, the main source really is taxes, which explains a lot of war in that Age. Crusades, The War of Two Hundred years between England and France, etc. Later at that time, the Queen quite some changes, one of them monopolizing the english wool for themselfs, and another investing on its industry. England now would win money by selling their wool to their own burgeois, which would begin produce cloth and other things in mass and export them. However we need something at the very beginning... Before the Feudal Ages, so they have something besides conquering from since the beginning. But what? Roman Empire... Persian Empire... All of them, focused on War.
Oh man, this is harder than I imagined.
Edit: I can imagine three things. One would be farming. Perhaps something similar to Greece: some territory is always dedicated to farming. A upgrade on Feudal Ages (since they focused on that) would increase the productivity of farming. Still, that's not enough.
Which lead us to our second idea: Resources. That's a really brand idea, really. And the way the games I saw are implemented I didn't like much how it worked. Except one. The idea is that you could control those and do a myriad of things with them: export them right away, use them to do more advanced material and them export that material, or when selling to your own industry you would win less gold, but the income would raise a bit (since you invested in industry).
Which lead us to the third idea: Trading. Classic Age of Empire 2 trading. The only thing worth mentioning since we all know it quite well, is that if it should be linked with the second idea (Exporting) or a independent thing.
Now, then, why would a "intelligent" just not do both?
Answer: Farms are quite expensive, and are only really effective after Feudal's age upgrade. (Although they starting giving some money already.) Infortunaly, the resources part does require a bit more of terrain. However, if you know to well use your resources, you can do a lot of gold by yourself. Also, should selling your resources to your industry increase your income on a fixed value or on a percentage value?
About the resources, consider giving a peek on this map:
Lords of Middle Earth
Perhaps, around each city (and a greater space if it is a capital) there is some kind of special terrain. You build your industries there (the ones used to manage your resources. Only one type per city, meaning if you build more you can have more and faster productivity... which is only useful if you have more of that base resource. You also build your farms there, and special defenses. Meaning again it is a choice between war and investment! The most powerful towers are those which uses one of that space, and also some kind of militia (a building with a skill to spawn several soldiers. And also be able to buy instant units there.)
Should a enemy country be able to intercept a caravan and sack it?
As you can see, at least on my point of view, it all fits really well. Russia had a huge territory, but a very... brand one. He didn't fill it with a lots of things, but still had a big population, meaning big taxes. Basicly what happens in LoE today. Then, we have... Pretty much, any feudal nation. At the feudal ages there were a LOT of nations, but they still had a lot of money because of their farming productivity. And battling each other, of course, but still handle themselfs because of farms, mainly. England's the perfect example for the 2nd idea. Well, I believe anyone knows why. If anything, just read above's Brainstorming. The third idea was really just a something else. Based on Reinassance Age, as the commerce begins to flow on Europe once more, and we discover colonies and begin to make calculations for economy, investments, etc. Pretty linked with the 2nd idea, actually. This part however can be improved.
Could be enough, as it is, in my opinion. Also, a litte tip on the 2nd idea: would GET the resources the Debei's way (resuming: like a city). OR each city comes with certain base products. Just one but with high availability, another city has two products with lower availability, whatever you decide. MANAGE or DESENVOLVE the LoME's way (too complicated to resume, but something like Castle Fight, but with items that represents resources, instead of units.) and EXPORT the AoE 2's way (obviously.).
A perhaps addiotional thing could also be... Improvements. You reach a goal, and win a bonus for that. Specialty: 6 industries desenvolving the same manufactured product. Win a extra source of that base product (or perhaps 20% bonus percentage of each source) and a bonus to income. Also: Diversity. Producing 6 different manufactured products. Win an extra by exporting and bonus to income. And missions, which would be so rewarding that could be a source themselfs. For example... Colonization. First step, building a great naval compound. Having lots of shipyards and ships. Second, technology. Have a quantity or perhaps percentage of total upgrades in-game. Third, having realized a economic improvement before. And finally, fourth: investing on the "colonization" itself (could be some kind of upgrade.) A colony would be a better import. Instead of for example selling to your own industry for 50% and win 10% of the total value of the item as income, you would sell for 100% and win 20% as income. The very first nation could get a little bonus (110%, 25%) and there is a number limit of colonies (4 perhaps).
That's just a example of course.
Hope you guys like it.
