The ending serves one purpose; to make you feel the way Peter felt

It's completely irrelevant to try and figure out whether Maggie really is from the future. The entire point of the ending (and the film as a whole) is to make you feel the exact same way Peter felt at the end, which is completely shocked and baffled. It's designed that way on purpose. Sometimes there's no greater meaning and no definitive answers. I used to believe filmmakers had a clear, singular vision in mind but I've read enough interviews and Q&A's now to know that sometimes they write movies purposely with multiple interpretations, leaving it up to the audience to decide. Don't get me wrong, it's fun to speculate because the film is purposefully ambiguous but just know that the real intent behind the ending is to make you feel the way he felt, and it succeeds amazingly well.