The first Expendables movie hasn't been released yet, but it's been garnering some well-deserved positive buzz from early test footage and trailers. So it goes without saying that they're considering a movie franchise. Producer Avi Lerner was already hinting at a movie sequel to the Expendables back in 2009:
More recently (in April 2010) Sylvester Stallone confirmed to Empire Magazine that we may well get ready for The Expendables 2:
Haven't seen the film yet so I don't want to speculate about the plot of The Expendables 2. But I guess that in the sequel we'll follow the next adventures of the surviving members of the Expendables team: the core members (Sylvester Stallone, Jason Statham and Jet Li) will probably show up again.
Unless they fasttrack the development of The Expendables 2, the sequel will most likely be released in 2012.
Update - A few plot details of the Expendables:
Who could join the Expendables according to you?