I'm really excited with how modding will be handled in this game. I had asked Paradox/CO staff on a few occasions about modding. Their answers have basically surmised to a wide array of aspects can be changed, and there will be two or three tools available to facilitate this, should the game stay on schedule. The first tool is, as Xannepan had said, a FBX importer for externally created 3D assets. The response was slightly confusing, but it sounds like there will also be a tool for creating assets in-game, if you aren't comfortable with 3D modeling in Google ketchup, Maya, 3dsMax, Blender etc. Nonetheless, there will at least be a tool for creating park assets in game. It seems like you'll be able to design a layout for a park in this tool using pre-made and probably imported assets too. This predefined layout will then be used as blueprints to fill whichever areas you demarcate as green-space. The third tool will be for creating mods that don't have to do with 3D assets; (ie. money mods, terrain texture mods, not houses and offices). EDIT: There will also be a map editor.
Overall, this is very exciting. I think we've already been promised/seen evidence of more of our wishes being implemented at this stage than we ever did before Sim City 2013's release.