SPOILERS. DO NOT READ BELOW IF YOU HAVE NOT SEEN THE MOVIE. I think you may have missed the point of the ending. I'm pretty sure they are not doing a sequel, and if they did, it wouldn't have Cobb in it (unless it's a prequel). The point of the ending was to make the audience member question what they were seeing as reality, thus going through the same exercise Cobb had been going through all film long. It was Nolan pulling an Inception on us. Depending upon what theory you buy, you may even think Cobb just pulled off an Inception of himself (to convince himself to exist in the dream state and accept it as his reality). But the last thing I think that the spinning top was doing was trying to set-up a prequel. It was a standalone movie - and is much more powerful that way. If Nolan were to explain what happens after the screen goes black, it cheapens the power of it all. END SPOILERS.