Campaign mode? It's a sandbox game, like the countless of other sandbox simulators, you make your own goals :)

Otherwise I guess you could ask people for the savegames of almost broken cities and try to rescue them?

Tons of sandbox games have campaigns. I think Roller Coaster Tycoon, for instance, was the perfect example.

Have you tried the mods yet? I had a lot of fun coming up with my own goals, like perfect happiness without cars (using only transit, bicycles, and walking paths), or flood-proof cities (the hydro simulation is pretty fun to mess around with), or rebuilding a city after an asteroid (or maybe fifty...) hit it...