A new SimCity mod detailed in a YouTube video and later explained in-depth on a Reddit thread reveals that it is possible to play SimCity offline indefinitely with a few important caveats. Despite those caveats the mod shows that the game can easily be transitioned from an "always connected" game to an offline single player game, despite what both EA and Maxis have said. According to modder UKAzzer he was able to do this with a little bit of work and by enabling the game's secret debug mode. This allowed him to turn off the disconnection timer limit. He explains it a little bit in this Reddit thread:
"Works a treat! No region stuff of course, and you have to stay in your city, and there's no way to save the city unless you reconnect... but it's something," he writes. "The '20 minutes' Kotaku reported on being able to stay playing for before being kicked to main menu for no connection is a random figure the devs put in, very odd."
So obviously the game would not be able to use the regional stuff and of course you can't save the game, but you can build an enormous city and toy around with it for an unlimited number of hours.
He also answers a question about how difficult it would be to remove the online requirements altogether:
"Hmm, don't know about totally, but I'm experimenting with hacking in ways to spoof it into 'claiming' a city without sending that info to the server, loading that city data up from local data, and enabling editing, so you can edit a city and pick a region square all offline," he says. "Still don't think I can find any simple way to SAVE it yet, but it'll be a start. Whether I take it any further than that, or leave that to the real hardcore coders/hackers with more time on their hands, I don't know."
Here's more on what UKAzzer did as shown in the video - from its YouTube description:
So with a little bit of package editing within SimCity, and a little playing about in the code, it's possible to enable debug mode. I linked the activation to the "Help Center" button in the main menu for ease. Most debug features are disabled without having an actual developer's build (they have terraforming tools etc. available in the full developer build!), but a few things do still work - including editing the main highways.
Not only that - but you can edit the highways ANYWHERE - even outside of your city boundary... and even if you quit the game and log back in later, it's all saved safely on the server.
This shows that highway editing will be easily possible, AND that editing outside of the artificially small city boundaries should be very viable too.
Other things I have modded out with a quick change: Unlimited time to remain disconnected (won't get booted at 20 minutes, can now be disconnected "forever"). Population count now shows REAL figure, not the "artificially inflated" figure. My large cities have a population of about 15k now, not 100k :P
We will have more on this story as it develops - including EA's response, should it issue one.