I know there has been a bunch of similar threads and articles, but here is my take on the topic:
- World Map - instead of having a bunch of separate maps, lets have one gigantic map, where you can place multiple cities. Of course, you would be able to play just for one city at a time, but you could interact with the other cities via the World Map: establish new road, train, ship and plane connections, exchange goods, plan tours, hold sports matches against each other... While you are playing for one city, the other cities could be on some sort of autopilot (similar to how households in The Sims work).
- Biomes and seasons - this has been on the table since the release of Snowfall. Let's make real seasons. With a "Biome editor", you could create various climates - 4 equally long seaons for the temperate climate, long winters and short summers for the arctic climate and short winters but long summers for the subtropical climate. But why stop at 4 seasons? We could introduce either a second set of tropical biomes with the 3 monsoonal seasons (rainy, transitional and dry season), or mix them alltogether in a Indian style 6 seasons year. Of course, the additional tropical seasons would probably be a part of a DLC
- Festivals - at the end of each season, there could be a major festival in town. You could create your own calendar and make each of your cities specific, with the celebration of different festivals. Some, that come to my mind are: Easter, Haloween, Christmas, Carnival, Day of the Dead, Lunar New Year, Ramadan, St. Patrick's Day, etc. Again, new festivals could be added either via mods or DLCs.
- Townhall - instead of starting a new city with a grid of houses connected to a highway, let's start with a townhall, town square and a temple. Again, you could choose from multiple different townhalls, which could then affect the visual style of your city - European, North American, Latin American, East-Asian, Middle-Eastern... (More DLCs, more town halls). And the idea of townhall ties to the next point, which is...
- Expandable buildings - buildings like the townhall, police station, fire station, hospital, harbour and airport should be expandable. It shouldn't be the style of expansion which is used for Parks, Universities and Industries, but rather something similar to "upgrading your base in a strategy game". E.g.: Fire station could become Fire headquarters, then a Disaster response unit building would be added, Firefighter heliport, Baywatch... Police station could be upgraded to Police Headquarters, added a Prison, Police heliport, Justice Court. Clinic would be upgraded to Hospital, then a Lab could be added, followed by Medical heliport and Gene therapy clinic. Upgrading townhall could be an alternative way of progressing in the game and unlocking new features.
- Roads, line and crossroads management
- Building of bridges, quays and dams
- Parking lots
- Mixed pedestrian and bike pathways
- Mixed RCI zones - (e.g.: tall buildings with shops and offices in the lower floors, aparments in the higher floors and a restaurant at the top)
- Curved and triangular RCI zones
- Better management of building styles (blocks of houses or individual houses, large yards or large buildings, skyscrapers or a row of middle-rise buildings)
- New park areas - like Spa and wellness
- Parklife-like sport areas - Ski resorts, Beaches, Racing tracks
- Reworked industries (no more RCI zoning for industries, but something like the Industries expansion with better integration of Fishing and maybe introduction of a new industry chain)
- Displaying individual underground layers separately
- Building the game in a more optimized engine
- Etc.
- 2