I am currently working on the new version, though it will take a long time to finish. I got lots and lots of other stuff to do which cuts my time down quite a bit.
I think the easiest solution would be do implement a linear loss in mine returns e.g every five minutes your income gets reduced by two. So this means that at the end your mines would only last for 1 hour and 40 minutes, configurable of course.
So now to the things I have planned in the future.
Currently mines are pretty boring, you just fly up to them, pay so credits and you are set for live.
Now the idea is to make every mine something special with you can interact and upgrade.
Upgrades would cost materials, credits, goods and time.
Some of the Upgrades I am planning include:
Production speed - (faster generation of resources)
Resource variety - (mine also generates resources of lower tiers)
Jump drive - (mine is able to jump to a neighboring system)
Market - (set products to trade with traders)
Upkeep reduction - (reduces the cost at which the mine operates)
I am open to suggestions as always.