Here's an idea: The bomb changes the past/future, but no matter how hard they try, the Losties can't escape their fate. With their memory of what had happened, the Losties can't NOT end up at LAX, and eventually the island. Some will follow their path willingly, others reluctantly. Many characters would reappear (Charlie, Boone, Daniel, Juliet, etc), but they won't be able to escape their fate and will eventually die again.
Its much like the course-correction with Desmond and Charlie. For example, Boone was supposed to die in the first timeline (which he did); Jack changed the past so Boone was alive again, but he will follow his path and die eventually. That way, any past characters can come back as guest stars but obviously won't be staying full season (unless someone can figure out who to avoid course-correction.
On a side note, ANY time-changing won't affect the people in the 2007 timeline. Because they're in the future, any changes in the past would have already affected them. Thats a logical fact, no matter what happens.