By now you know that Walt didn't return this season. But he really should have. This is how I would have brought him back:
During one of the flash sideways (or what appeared to be a flash sideways), Walt would be seen at a funeral. The body wouldn't be shown but Walt would appear to be extremely upset, piquing our interest.
First, we'd wonder when and where he was because Walt wouldn't be this old in the sideways world we're familiar with, which was supposed to occur in 2004. Second, we'd wonder if this was a flash forward from his first life, and if his grandmother had passed away. Third, if it's not a flash forward, we'd wonder if it was a flash sideways, how many years had gone by, and whether or not it was Michael in the coffin. (At this point in the show the sideways revelation wouldn't have happened yet, so we wouldn't know that it was a pathway to Heaven. We'd still be thinking it was a possible second chance.) In a matter of minutes, the show would have sparked a new mystery that would have made the fans go crazy.
My conclusion: since time does not matter in the sideways world, Walt's age doesn't matter either. To me, he simply manifested himself in the sideways world – in this form – because it was at this stage in his real life that he started to believe his father would never come home.
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