I am not sure if this has been suggested specifically before, but I would like a tickbox adding to each mine/field where once ticked that mine/field can go down to 0 without it becoming exhausted. The reason for this?
1) For mines it allows you to have higher level mines running without the risk of them getting destroyed during downtime/maintenance or if real life takes over and you cant get back in time to rescue said mine. Level 1 mines can be left unticked so they function the same way they do now. This means the high level mines can have a deposit added to them after they have run out.
2) For fields this would make them function similar to how they work on the test server whereby silos can keep filling up nearby empty fields, this makes the silos a bit more valuable if you don't have enough production from them to stop fields ever running out (applies to 99% of people with silos). It also helps farms keep a small amount of production even if you manage to clear out all your fields (ie if you don't get an opportunity to keep up with replacing them).
This post has nothing to do with minefields, I am not after explosive mines or minesweeper units being added to the game :P