We'll be right back after these messages...
I think I better state my points...
Rinoa is the Final Sorceress at the end of the story, as we as far as we know it. And with Ultimecia (her possible and I use that term loosely because she may not be, future-self) still existing in the future Sorceress Rinoa is subjectable to possession at any time.
Ultimecia is possibly, technically within the reach of the characters lifespan of years and the Sorceress Power, having a self-preservation, mutates physical characteristics of it's hosts which may also include prevention of cosmetic aging...
It is uncertain that Rinoa will have a successor simply because it has been 'tradition' of the as it were.
I have to go now... running late..... But we'll be back with more after these messages....:D