Though for a time it seemed the Wanted 2 producers would finagle some way for Angelina Jolie to return for the sequel, Jolie has been saying firmly but politely throughout her Salt press tour that, no, she can't return for Wanted 2, because her character is dead. It seemed like a case of everyone involved being willing to cut their losses and move on without her, but as Mark Millar tells Digital Spy, it was more her decision than anyone else's:

"I think they're regretting killing Angelina Jolie off, although that was a great scene. They tried to write a thing where she came back from the dead and Angelina was like, 'That's terrible!' They're trying something else."

He also claims to have no knowledge of whether or not Kristen Stewart will be stepping in to replace Jolie, which I buy-- there's no reason for them to keep the writer of the original comic in the loop on something like this. It seems to make more sense to ditch Jolie's character entirely and let James McAvoy handle things on his own, since that requires a lot less suspension of logic. But hey, Wanted has kind of always been about ignoring logic in favor of awesome shots in bullet time, so anything is really still possible at this point.