Honestly, Improvements probably need an overhaul on how they are handled in general. As I have stated several times through this forums, improvements like most things in Elemental lack any element of choice. Consider that, the only real limitiations to buildings are tech pre-reqs, resource pre-reqs, numerical limitations, and building reqs. Yet, the tech reqs are simply a method of limiting how fast you can start building improvements and resource pre-reqs are a type of geographic determinism, which forces players to expand to acquire these resources. Even, the building reqs are simply a set of building which follows a very simple OR logic. From my point of view, improvements should have a broad set of limitiations which can be used. I think the following would only enhance our ability to mod improvements:
1) Allow for improvements to use a complex logic of reqs for determining possible building. Currently, Building reqs are simply OR'ed to figure out if any of the pre-req buildings exist. It would be better if I could specify that Improvement A can only be built if Improvement B and Improvement C exist or Improvement C and Improvement D. Even further, we could say something like A can only be built if B and C and not D or not B and C and D. This would allow modders freedom to force players to choose how they improve their cities in such a way that is ultimately meaningful.
2) Allow for Improvements to have multiple upgrade paths. Currently, if you have Building A, you can only have it upgrade to building B. It would be more interesting if one could say, you have building A which can become B or C. You could even go further to say add a more complex logic here as well to say, A can become B under condition X or it can become C under condition Y.
3) Allow for production times to be variable based on conditions. Like, if you have building A, then building B can be built in only 50% of the time, or to go even further, if you have building A and B, then C will take 2x as long while A or B, then C will take 0.5x as long.
4) Allow for complex conditionality for pre reqs. Right now, all pre-reqs are based upon simple and logic, so that all pre-req conditions must hold. I think it would be interesting to allow improvements to again specify complex conditionals for this.
While I know this would greatly increase the complexity of the improvement system, as it stand now, the system is highly simplistic and building cities feels ultimately linear.