I don't know whether this has been asked before, so please correct me if I repeated an old question.
How are you going to handle research and technology progression?
There are a couple possible models. Investment can be done on a per-time basis or in a one-time manner and the effect can be equally either continuous of in discrete steps.
An example of discrete investment and discrete result would be the technology tree from games like Age of Empires or Starcraft. Invest 500 gold now and receive a 10% bonus to building defense. Then safe money for a while and buy the next tech.
An example of continuous investment and discrete result would be the technology tree from games like Sid Meyer's Alpha Centauri. You allocate a percentage of your resources to research and once in a while you get a pop-up that says that your scientists had a "significant breakthrough" and you receive a certain bonus from then on.
In a continuous investment and continuous result scheme you receive advantages like in the game Ultima Online. You invest time by practicing a certain activity and slowly improve your character's skill level and thus slowly gain bonuses to these actions.
The first model is the easiest to understand. The second model allows chance to influence your progression in technology. A lucky breakthrough early on can influence the whole campaign. However the tech-tree gets very complicated because everything has to be modeled in discrete steps. The third model closely resembles almost all research done in the automotive sector (evolution rather than revolution), but abstracts away the technological details (you don't get a popup saying you now have super-cool aluminium space frames). A mixture of the second and third model gets rid of most drawbacks but adds complexity.
Have you made up your mind about this already?
I'd go for a mixture of the second and third model since your target audience likely wants the complexity, but it's your game so you decide.