The X could signify an alternate dimension. For example, our version of Los Angeles would be LA prime, a new version could be LA X (or A, B, C, etc.). The thought being that while Jack's intention was to reset their own history with the Jughead detonation; the destruction of the Island in 1977 could set a course of events that radically changed all time and history. So Oceanic 815 would land in Los Angeles in 2004, but it would be a completely different version of Los Angeles than they expected. In comic books, "X" is often used to designate an alternate universe. Both Marvel and DC have used "Earth X" in reference to parallel worlds, so "LA X" could mean an alternate version of LA.