Ok, I thought of some stuff that may fall into the theory that Rinoa eventually becomes Ulitmecia.

1. When Ultimecia posesses Rinoa at the end of disc 2 which leads to Squall and Rinoa going to the space station and then freeing Adel. What I beleive happened was the Ultimecia possesed Rinoas body and then she then lets go of Rinoa and possesses Adel, which would hereby releve Rinoa of her Sorceress powers henceforth not being a sorceress anymore

2. Rinoa witnesses Squall kill Ultimecia at the end of the game, if Ultimecia wen't through all of that trouble to plan to possess people and try to take of there world with here powers then if she were Rinoa then she definetly wouldn't make the same mistakes twice and remebering all of the stuff that Squall and his friends did to thwart her plans then hereby not failing in her plans.

If Ellone would happen to be Raines daughter then I can add more to this theory but untill then I can most likely say that Sorceress Ultimecia is in some relations with Ellone, Raine or most likely is Ellone herself.