Add the ability to excavate for freshwater man made lakes and reservoirs.

Add the ability to excavate for freshwater man made lakes and reservoirs.Which could be constructed from a range of materials, such as rock, earth, concrete or Plastic liner An earthen embankment could have a watertight concrete face or a core of clay which could be found excavating down to the games already ground water levels. A new pumping station could be added with a water treatment plant.The reservoir could also be filled with water that has been desalinated from the sea. And of course rainwater.

Fresh water would be very useful but for realism I think water would also need to get polluted, so you cant dump sewage and pump out water. I dont know if the game mechanics allow this.
You can make salt water lakes easy enough. Dig out the lake then link a shallow dyke to the sea to fill it up then fill in the dyke.

Empty mines tend to be nothing more and a rock bowl, meaning that they could serve as a reservoir location. However, choosing a more suitable size and location would be better.
Once a suitable location is mined to the rock bed, there should be no need for a "liner. However, should it be decided that it would require something, using system similar to the "Surfaces" painting system could be used to apply the manufactured surfacing material (concrete would suffice as "plastic" is far too late in the tech tree), and the ability to build the reservoir should be soon, and inline with, the "retaining wall" tech. Like with both of the Seawater Pumps, and the CO2 burying process, a minimum depth should be required (and wanted).
The "liquid dump" and the extraction pump could be a new structure(s) as the current structure(s) would otherwise need code\graphic modification(s)

This would be quite useful in maps like the crater one, where you could just fill the crater up with water once all the oil is extracted.
For any number of things too. Cheaper liquid storage... Stock it with fish, if that gets added... Pumped storage for hydroelectric plants...