So this is basically what I got out of the series.
The island and what happened on the island happened in the real world, so they weren't dead. The island was basically a device to give them a purpose in life, if you look through the series it's not really about the mythology, it's about the character development and growth. In the end the island helped the main characters find what they were looking for, gave them a chance to right wrongs, commit wrongs, and find ways to repent.
The flash sideways was a purgatory-like world that the characters created (Christian stated as much) meant to reunite the characters. The island gave them a purpose, and that purpose was one another. So it's only natural for them to want to reunite as a group before they move on to whatever is next for them. In the end their "heaven" was finding each other, remembering what they had gone through and reuniting.
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