I think this whole thing is just going to be Hurley's time in the mental institution.........this is all just made up in his head. IMHO.
This had better not happen. Would be the WORST. ENDING. EVER. I've already seen the ending of St. Elsewhere's once, thank you very much.
It seems like at this point if they brough Walt back in as an important part of the plot it would feel a little contrived since he has been off the show for so long, but who knows. I think we'll see him at least once more in the flash sideways though.
I'm thinking there will be plenty of minor questions left unanswered.
You know, I love the show and have watched it religiously since Season 1, but I do get the distinct impression at times that in the beginning of the series the writers and directors threw so much crap against the wall and only now are they beginning to sort it out, while leaving a few things unanswered.
Walt was set up to be an important character at the beginning, and now he is an afterthought, if even that. I wouldn't be surprised to see him make a reappearance here, but wouldn't be surprised to not see him mentioned again either.