The biggest mistake they made with SimCity is that it was 100% online, and their servers couldn't even handle it. It took them a year to reluctantly allow offline access after complaining it was too difficult to do, but it was too late, fans had turned their backs to them.
I think Minecraft is a good example of how a game handles both modes, it can be an exclusively offline experience but you can create a private server to play with your friends.
I personnaly have no interest in multiplayer gameplay, so my worries would be:
- if there is an online component, that its development does not mean half-done single-player mode.
- if the two modes exist, that they will be completely independant
