There has been no official word about which world will be coming out next. Valencia is a possibility along with Krokatopia, Darkmoor, Grizzleheim, along with expansions on worlds/systems previously explored.
Yup, everyone has their pet theory. Valencia is certainly possible, but to the best of my knowledge KI has never said what it will be. My personal theory is Polaris, based on some admittedly scant evidence!