I have a small idea, sorry if someone has posted the same idea before, but i don't have time to read all the 20 pages of replies here.
Here goes my idea,
Many of us always have wanted to "walk" inside our cities, like CityLife and SC Societies, but my idea is different, build the city as we have been doing now, in iso metric views, and realistic graphics, (with or without gird, this doesn't matter, only I want is to zone diagonally too), then we can have two possibilities, one could be to zoom in our city, and walk (or fly) inside it, in a mid-graphics detail enviroment, so we would not have computer specs problems, we could walk in front of our nice house, then we can take a photograph of it and then the game would render it in hi-graphics, so we would see our house detailed, with shadows, reflections, etc. The other idea i have is to skip this "low-graphics" walk and directly choose a camera position inside our city and take the photograph anyway. To accelerate the rendering, we could have some pre-rendering options, like day time, weather, details, refletions, and all this kind of options to make look nice our photograph.
I wish you understand my idea, and sorry again if someone had posted the same idea.
Other ideas i have are about the gird, i like to make "squared" cities, but also to make some curves and diagonals inside it, so would be possible to activate and deactivate the gird for build freely, or make the zoning gird as the road next-to it.
The possibility to make airports and seaports with unique shapes and sizes, like we had in SC2000 and SC3000 editions, and some SC4 lots.
Of course the great work of NAM should be there, but with enhanced options.
Making tunnels easily, you choose where the tunnel starts (like in SC2000), but then you can choose where you want to place the tunnel exit.
I have a lot more ideas here, but these are the main ones.