So you have tired of the usual discussions about Titanic and that very famous movie and are looking for something new. In a Castle like moment, it happens! Jack Dawson was a time traveler sent back in time to prevent Rose from committing suicide. Shazam! The Internet goes ablaze as people discuss its possibility and offering proof. Like the famous cursed mummy, it is sure to last a long time.
If we follow the logic of its believers, if Rose had fallen overboard Titanic would have stopped to look for her. This would take time and quite possibly prevented Titanic from colliding with the iceberg. History would have been changed but for good or ill we do not know. Like all such theories, it opens up a lot of possibilities for alternative history writers. If Jack came back in time to save Rose though it could only be for one reason–that something very bad will occur as a result. Unless of course there was something very personal he wanted to change but it likely would be the first reason.
And Rose is the key. If she lives, history will be changed as Titanic will sink. Some point to the manner of his death as another indication he was a time traveler. Or that he references places he will take her that do not exist yet. Well it is certainly interesting and even entertaining to think about. Which makes her more important than Indiana Jones was in Raiders of the Lost Ark. In that movie, as those guys on Big Bang Theory realized after Amy pointed it out, Jones was not central to the story at all. If he had never been in the story, the exact same ending would have occurred with all the Nazi’s being killed in the end just without Marian and Indy tied up nearby with their eyes closed.