Stability would be paramount in a sandbox game like
SimCity 5. Given the amount of effort that each player puts into the city (or cities) they develop, crashing is like someone kicking your sandcastle over. The developer has to take care not to release a buggy game and patch it later, especially if it affects preexisting cities. It goes without saying that downloading a patch and installing it takes timetime away from the game you just paid for.
The Sims was an example of this. Too many bugs only annoyed consumers. Though sales didn't appear to be affected by this, the game is still known for being poorly made. Something as complicated as this game is likely to be, it's going to be frustrating enough if people aren't careful in designing the gameno need to make it worse with unnecessary crashes and bugs. To release an unfinished game gives it an unnecessary tarnished name.
Something else that would keep people playing for a long time is the ability to earn things. It goes without saying that the developers should allow for a Sandbox-type game mode where everything is served to the player at once of course, but in the main oneand in whatever campaigns there may bedevelopers could allow for players to unlock special buildings and earn a variety of doodads.
Actions should have consequences, and a thoughtless mayor should face disaster. A good mayor would do all he or she could to avoid them. This could serve as a challenge to players, in addition to the other logistical nightmares one faces. The game should allow for some warning, as something that experienced players can keep track of but something that a new player might stumble upon as a surprise. Always put in an edge of uncertainty, so they aren't completely predictable.
Compounding on certain unlocks offered by SimCity 4, the game could allow for players to start in the present and accrue a certain amount of revenue before unlocking a giant statue or some other mega structure.
In SimCity Societies, you could pick the flavor of your citysome funny and some more serious and dark. Most of this could be settled between different tile sets and written content, so such features shouldn't be too difficult to recreate in SimCity 5, so long as the developer doesn't reproduce the "casual" nature of SimCity Societies.
