We want to be honest here - it's definitely not designed for it. With some adjustments the game could be solo'ed if need be. You'd have to pull out some easy quests, because you'd be working together perfectly on resources instead of out auctioning each other for V.P's. We can't it'd be the ideal experience, but hey, sometimes getting a group together's tough and you just have to run it solo.
We're going to talk to the designers about some variant rules possibilities. This question has come up a bunch and we will try to do an update or have a conversation on bgg where the designers can say what they think.