I'll just use process of elimination here:
1. Both realities will continue to exist and at least some will exist in both
- Too inconclusive and a cop out. Won't happen.
2. The Alt. Reality will be the final reality (island reality disappears)
- Fans riot after realizing they wasted six years watching an irrelevant reality. Won't happen.
3. The Island Reality will be the final reality (alt. reality disappears)
- Fans riot after realizing the final season was pointless filler. Probably won't happen (wouldn't be as aggravating as 1 or 2, so I'll give it some probability.)
4. They end up in some new, merged hybrid of the two realities (a new reality)
- The minds of their dopplegangers in the "zapped" reality merge with the minds in the reality that stays so that if Ben learns to be a decent person in Alt-Land, he'll retain that learning in the Island reality. This option is the most likely to happen but the problem is that it's corny and predictable.
5. Some other (specify)
- Since I dislike the other four options, I hope this is the one that happens, but I have no idea what it could be that wouldn't be even worse, such as purgatory, hallucination, or other unwelcome notions.
One thought for Door #5: if it somehow included the notion that the 815 crash that Widmore elaborately "faked" was an authentic Flight 815 after all, with everyone onboard (all the characters in one of the realities) dying, that could be pretty cool, depending on how it's handled.
Not according to the producers. Lindelof and Cuse have said unambiguously that the flash sideways are not an alternate timeline in any sense, and the characters are the same characters.
Did they say timeline
and not reality
? If so, that could be an important distinction.
I'm still clinging to the theory that the flash-sideways are going to happen as a result of whatever happens in the finale
If the flashsideways are the future of a single timeline, then why don't people recognize the famous original survivors of Flight 815? Maybe the characters get amnesia in the final ep, but the whole world can't get amnesia.
The flashsideways could be the future in being the afterlife. That would explain amnesia for the whole world.