The last title in the SimCity series, SimCity Societies, was the first to introduce a fully 3D environment to the city-building experience. The next game will remain in the third dimension, with fully 3D spline-based paths for you to build upon. I would guess that Maxis will be introducing 3D camera navigation as well, which Societies was notably lacking.
But the next SimCity is no casual gaming experience like the poorly regarded Societies was intended to be. It's been built from the ground up, starting with a new data driven simulation engine built specifically for Maxis games. It's called Glass Box and it includes resource simulation, individual units, and maps--all shiny and freshly reengineered.
Maxis has stated that the new SimCity game will be capable of supporting tens of thousands of individual units. Discrete people and vehicles will all be able to roam your city on their own, each following their own paths according to the logic of the simulation. All in three dimensions, all fully rendered.
And because this is 2012 and everybody is online one hundred percent of the time, the new SimCity will be hooked up to the internet. You'll be able to save on the cloud, meaning that even if your laptop is tossed into a ravine full of very sharp rocks by a vengeful adversary, you'll still have your precious city when you boot up your backup machine. You'll be able to keep tabs on your world via an external browser, too, in case you've left it running and want to check in from your smartphone while at the gym or wherever else you go in the overrated real world. There's also multiplayer, and the way the server is set up, you won't even need to be online at the same time as your cohorts to be able to play with them.
Maxis has stressed, though, that the next SimCity will be much in the spirit of SimCities of yore; a largely single-player experience and a gleeful time-suck. The game is currently in development and is scheduled for a 2013 release.