If you any of you guys have played EA's other games you would see that custom content can function across users even without the files downloaded locally of everyone's hardware. I play the FIFA soccer series. Custom content (teams, players, logos, ect) all work just fine when playing online. The other player can see my custom content. It's possible. I just don't know how it works. I'm certainly not a techie.
Perhaps modding will be limited, and the modding tools will all be online tools. I'm just certain that these tools will eventually become, and EA will probably charge a fee to access them. This is just how EA operates. They might even incorporate some way for users to design custom content that can be traded for "coins" so you can "buy" someone else's custom content. Or you can just buy "coins" with real money.
This is all just speculation, but I'm trying to take what I understand about the online component of EA sports games and apply that to SimCity.