It represents everything the people fought for: peace among the kingdoms, returning to life as usual, rebuilding, taking on new roles, also shows just how much everyone has grown through their experiences. Arya once considered sailing off, but instead chose to protect her family-- she sailed off in her own ship with crew. John is not king of the north, but is respected and welcome in the area and has his wolf. Sansa is queen of the north, having refused to join the other kingdoms in unification. The gathering at the table (Tyrian and company) shows that there will be challenges ahead for the new unified kingdoms, but trade and mutual benefits to all give hope for the future. Everyone's story led them to their new places of power and responsibilities.
All of this is in contrast to the endless wars that would have occurred if evil has been allowed to reign. In a way, it says that everything they all went through was necessary and worthwhile.
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