Xenosaga was supposed to be six games, the first 3 being a trilogy in that it featured the same cast. I think the plan was for Xenosaga IV to take places several hundreds of years after III but Namco stopped the series after III apparently due to not meeting sales expectations. From what I understand, Xenosaga basically rebooted Xenogears which, strangely enough, was episode V in that series. Okay, it's not really a series since it's only one game, but it was planned as a series.