The biggest unrealism factor in all city sim games is that building happens instantly. I suppose it has to be there just to make them playable but I'd love to see a city sim game where building proposals get stuck in committee for years, challenged via environmental review, cut down in size and scope and eventually built as a compromised version.

Part of the difficulty of urban planning is that the feedback loops are so long. It requires you to plan 5 - 30 years into the future and mistakes are hideously expensive. Finding a way to model that would add a new dimension of depth to city sims IMHO.