Well there was Streets of Sim City[0] where you could take any of your Sim City 2000 cities, and turn it into a vehicular racing and combat playground. Way fun at the time!
The thing I remember most about that game is dropping wounded passengers because my heli couldn't keep the load up in a storm :-)
I loved the in-game radio and the enormous number of goofy/awful commercials. It was a huge amount of fun to build a city in SC2K or SCURK, fly through it running missions, and then burn it to the ground with an Apache "fighting crime" (aka clearing traffic jams the easy way).
Loved that game, it was for me like a light hearted version of GTA without all the dark overtones. That's really what would make SimCity a game I would have kept upgrading to; that and support for Linux. Their main page says PC/Mac. So PC means Personal Computer, wonder if my PC running Ubuntu counts? I'm guessing no. I'd have kept upgrading the BattleField games too but EA seems to not be interested in this.