If the devs want suggestions, they need go no further than Sim City 3000 which to me was their best work. We had the BAT which allowed us to develop our own structures. We had terraforming where we could do our own splining, and it allowed us to remove a hill if we chose to do so.
We had two different looks to our city: One was the above ground look we always have but the other was the underground look of water pipes and subway tunnels. I thought the subways should have remained.
In fact except for the sanitation, we had it all. We could even build expressways, another thing they removed from the game.
When it came to power, what happened to the gas plant and the fusion reactor that we had with SC 3000?