My ideas about how should SC6 look like:
You can choose the type of regions:
every type could have some specific elements you could add only into that type of region.
you would start in fully customizable and growable region or from various regions based on real places of the world
The development of your city would be divided into levels. The more numerous and more happy your population is, the higher level you reach and the more stuff you unlock.
You could choose the look of every public building (Police stations, Fire depts., schools, etc.) in various styles, for example:
West European (old or new)
East European (old or new)
North American (old or new)
South American (old or new)
the higher level you get, the bigger building with the bigger effected area could be built
there could be an option to make every public building accesible for people with disabilities (it would cost some money but it would also positively affect the mood of neighbouring citizens)
The RCI zoning (as known from SC4) could be applied. In Level 1, only old-looking buildings could be built (you could choose their look from styles mentioned above). The higher level you reach, the more styles of buildings would be unlocked (all of them in style that you would choose).
The transportation stuff:
Roads, streets, avenues; railroad, small passenger rail stations, small freight station, tramway, tram stations; bus stops unlocked in Level 1
there could be possibility to arrange the bus/tram routes
Motorways, bridges, tunnels, pedestrian zones, bike paths; larger rail stations, bus stations unlocked in Level 2
there could be possibility to forbid entrance to the city for trucks (as in real cities), to integrate bike paths with streets, roads or avenues or make them separate
Underground, elevated / mono rail (with their respective stations); passenger's & ferry docks; small airport unlocked in Level 3
An international airport would be unlocked, when population and mood of citizens reach some limit
Electricity - the higher level you reach, the more efficient and more clean power plants you could build
There could be possibility to build a hydroelectric power plant (a dam)
Water - the same as in SC4
There could be possibility to build a gas pipelines, gas refineries etc.
There could be possibility to build an internet cable network (unlocked in higher levels)
I have a lot ideas and later I add them here.