When it comes to puzzle games, the Portal series is there at the top. The first game was a resounding success and the sequel only strengthened that success. Chell's story may be over, but Aperture is still there, and it would make sense that there would be other test subjects within the vast complex. Even though there were 5 years between both games, with today's technology it wouldn't be silly to hope for a sequel to be announced this year. Valve needs to start doing what other companies are and milking their franchises, at least until we get another Portal and Half-life game.