Yes it was opened ended. We got very vague ending. All we really know is that the Holy spell stopped Sephiroth's Meteor spell. However, The ending never really say what happened to Cloud, Tifa, et al. The only character that had anything shown was Red 13, and that was far into the future when all the human characters are long since dead except maybe Vincent.
Constast that to , FF4's ending. That ending told what happened to every character you ever played, even that wimpy bard prince.