The ending of Rebellion screamed sequel bait : with Akuma Homura's universe is presented as unstable, Madoka who can start to rejoin her goddess side at the minimal trigger, Sayaka swearing that she'll forever remember that Homura is a demon, the later promising that they will have their fight one day and her comment to Madoka that sooner or later they'll becomes enemies, the only thing that lacked was a big "To be continued in next season/movie" after Homura jumped off that cliff.
Even the creators admitted that they made that movie with the objective to continue the story. The only questions left are the format it'll have (movie of series) and when it'll be released.
Although I admit that it would had been very ballsy to end the story here, with the now übermensch Homura bringing down a goddess and rewriting the universe following her wishes. It could had been a metaphor on how mankind got past superstitions with the power of their will and the dare to act against long established conventions or something like that!
Although, I guess I'm not overly surprised. It's got a big fanbase. (That, and animation is treated differently in Japan than in the US.)
