Of all the big PC games people are playing at the moment, the one that needs a mod like this the least is probably Cities: Skylines. But then, that's also why it needs it the most.
Normally, Skylines is a game where you hover above the world like Civicus, the God Of Urban Planning. Fr0sZ's mod, though, not only puts you in the shoes of one of the game's tiny little citizens, but lets you do so with someone else alongside you.
Before you get too excited, know that at the moment, that's about all it does. Fr0sZ is soliciting ideas on what you could actually do together in a city, and while there are already a ton of predictable responses like "driving!" and "shooting!", driving would actually be pretty great.
Driving or gardening.