The first film's not even out yet, but series creator Mark Millar is already talking about a WANTED 2.
Via his message board:
PS Wanted 2 already being planned and they've asked me how I can develop some of the other stuff from the book into the sequel. We'll see what box office is like at the weekend, but everyone knows this is going to make a LOT of dough.
...
Wall-E permitting. Fucking bastard of a wee robot.