I don't think the success of games like Kotor, Fable or oblivion can tell you anything about how the western-dominated market of the Xbox360 will respond to SE's much more easternised RPG offerings.. The style of games are vastly different and cater to very seperate tastes on the whole..
Granted FF is a massive IP globally and would probably sell well on the platform but well enough to warrant a port of SE's biggest franchise in the core offering?
Big franchises are the easiest to port and have financial success especially global IPs such as Final Fantasy.
There are alot of variables and SE have a history of being extra catious with respect to where they deploy the biggest franchise they have which represents the heart and soul of their company.. And besides, I doubt there relationship with Sony would benefit from losing FFXIII's exclusivity since what would that say about SE's confidence in the PS3 especially in Japan when the Xbox360 market is practically ignorable..?
SE wasn't cautious when it dropped Nintendo and went to PS1 it did so out of financial and technological reasons. SE main motivation is to generate profits. 92% of shareholders are not going to let the remaining 8% stifle profits to serve the self interest of that 8%.
I highly expect SE to go the route they have so successfully followed in the past with regards "testing the waters" by building an original IP for the Xbox360 over porting the core FF, possibly (no... probably) within the Fabula Nova Crystallis universe.. Or they may decide to port the FF Versus XIII title over which they have already stated would be much more real-time-action-orientated (I assume it will be similar to the Kingdom Hearts series, especially since it's been done by the same team) which, as an RPG staple, seems to go over much more favourably with the western markets....
What original IP did Square release prior to releasing FFVII on the PS1? How is the popularity of an unknown IP going to help you guage the the popularity of your biggest franchise on a new platform. Name a company or games that have used such tactics.