Not sure if this has been requested or not before but one of the first things that annoyed me in the game was transportation costs. I don't know any city that has free transportation everywhere. I'd love to see a mod that allows us to offset the cost of transportation...bus fares, train and air travel fares etc. The rides per day or hour are tracked so there has to be some way to use this mechanism and charge a fee. I mean they made this nice graph so you can see what your operating costs are with a side that shows if you are making profit off that particular group so I'd love to be able to dig into this.
I'd even start digging into this myself if someone can point me in the right direction.