Along with the various aeons becoming the Final Summons and getting killed by Yu Yevon, it makes me wonder what forms the OTHER Sins were when they were out rampaging.

It is implied that the Final Summons is permanently destroyed when it kills the prior Sin & takes its place - to judge by Lord Zaon's empty aeon statue. Since there are a lot of summons from the other Final Fantasy games that don't show up in FFX, it makes me wonder if some of them have been used up as Final Aeons already. If so, then IMO Jecht is inhabiting Leviathan. ^_^