With the references to Momento and the filming backwards theme, I ran a hugely successful campaign for almost two years based on:
1 - the characters in the campaign were "time twisted", The PCs started the campaign literally under attack from powerful forces - and spent most of the time on the run from powerful enemies thereafter. The PCs had *no idea* at all why these villains even WERE their enemies. As matters progressed, the PCs discovered things that ancient heroes had done long ago in the past to these enemies and assumed they were these Heroes' reincarnations.
A dozen or so sessions in, it began to dawn on the PCs that they were not reincarnations of these Elder Lords, they were - in fact - the Elder Lords who were somehow Time Twisted. The "past" to the game world was the "future" to the PCs. The PCs were thus being pursued by ancient enemies who were trying to prevent the PCs from doing something that had already done - pivotal events in history that the PCs had yet to do.
The whole aspect of being Time Twisted was very difficult to juggle - but in essence - it was very much a campaign where the story started in the middle, and the PCs were trying desperately to figure out what was going on so they could get back to the "beginning". In 25 years of DMing, it was easily the most successful campaign I ever ran.