Another silly game idea
Effectively, Sim City meets tower defense, where you build a city and defend it. These sort of exist in some forms, where you build a complex of buildings and have to defend it. However, I’ve never seen one of those that actually deals with buildings and roads as actual things. Vehicular units are stuck with using roads, and can get choked at bridges. Foot units can enter buildings and move through blocks.
The really fun part would be mapping the engine for this onto map data from a real city, so all the roads are real, and you get to run around blowing up someplace real. This part of the engine isn’t even hard, provided you can get the topology of the city and render the map on top of that.
Admittedly, the way this came up was that I was thinking about a browser game where you defend the city of Toronto against the suburbs, and so the attacking units are bankers and soccer moms in SUVs, enemy waves are called “Rush Hour”, a biker can one-shot-kill a cab (and vice versa), you can build the “Gay district” structure (gives you a once-per-year freeze-all-traffic event), or the “Hacklab” and “Site3” structures (which pump out infrastructure hackers and flamethrower troopers), etc.