Didn't play Far Cry 3, but I did play Blood Dragon, which was awesome and stupid and fun. But I would say that much of the enjoyability came from the fact that it was so unexpected and different. I doubt a sequel would be able to surprise people in a similar fashion.
They should make a spiritual successor, though. Completely different setting but same general idea of over-the-top sillyness and quirk. They won't be able to escape the expectation trap, though, and knowing what kind of whiny assholes the loudest gamers can be, there will be disappointment.