SimCity is a good example of how NOT to do it, IMHO. I think the market will punish them for it, though I could be wrong.
With Diablo 3, the goodies that you find are persistent. So it makes sense that its designed as a persistent online game. Solo players may disagree with that decision, but it allows any character to jump into anyone else's game, and it allows anything you find even while playing solo to be sold online to other players (eventually for real money.) Basically it attempts to fix D2's multiplayer, which itself was quite popular but was easily exploited.
As a solo player I'm not convinced it was a worthwhile tradeoff, but at least it's a valid design decision. (Interestingly, Blizzard is undertaking to track hundreds of millions of these persistent objects on their own servers, even though there's no MMO subscription fee. But that's another topic.)
What little I've seen of the new SimCity doesn't justify the online requirement. So they can gather stats? Big whoop. If there's no payoff for players then it's bogus.
In addition, while SimCity is a time-honored title, I don't think it has Diablo-level anticipation for the next title, and (unlike D2) nothing that demonstrates demand for online multiplayer.
I could be wrong- they could be doing a design from scratch that makes multiplayer a fundamental part of the SimCity experience. For example, instead of advisors, real people co-governing a city. Maybe requiring bargaining between the parties over limited resources, as well as disasters that require multiple hands to effectively address. It would have to be pretty deeply embedded in the game to mean anything, though.
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Just posting for those who think this won't become the norm with AAA titles within the next 12-24 months
The sky is not falling. I believe that companies that think that D3's model can be slapped on for Instant Profits are in for a rude awakening by the market. Not to mention the added expense of running large parts of the game on servers instead of the clients.. I think many companies are smart enough not to blindly follow suit in the first place. Even Blizzard is unlikely to adopt this model for all their games.