I like the idea of degenerating mines.
As a start one could keep mines as they are today, where you get a quick peak before it drops down to a certain level (where it stays forever). From that level you could get a slow decrease down to 10-15-20'ish ore per truckload. (Mines in real life is rarely completely exhausted, they just become unprofitable to run I think)
Personally I'd like to think if you combined the decreasing ore with some alterations(increase) in high tech unit costs, one would see a very different scenario when it comes to battles than what you see today (=Mass spamming of best units and forces meets head on in battles of attrition). This way it would be possible to weigh many cheaper units vs a few high tech units. And other tactics would be of higher significance (Unit movements, Walls, GP's.. etc)
With this change, the unused Midas and Resource Race might also be viable game options ?
If one decided to make this change or to test it - why not just make an "expansion" or how one would define it - where you could just create a 2nd copy of OP2 and have these changes in a modified version, while still having the original OP2 on the side..