Maxis' freshly announced SimCity, as with its Sims series and previous City games, should have plenty of mods available post-launch; the developer noted at GDC the new GlassBox engine the game is built on has modding in mind.
"We're huge fans of our modding community," creative director Ocean Quigley told the audience. "We've designed all this stuff to be moddable."
"We know modding is hugely important to our community. We know the reason why people are still playing SimCity 4 ten years later is because the modding community has kept it alive. GlassBox is built to be moddable, but beyond that we haven't announced anything."
If you're a modder yourself, it's said the package format will be the same as with The Sims and SimCity 4, so working with the new data should be an easy enough adjustment.