The only thing I can think of that could be possible, is that Seifer is punished for his crimes and is put in prison.

Rinoa can't control her powers and slowly goes mad and starts harming people, that she refuses to harm at first, but the insanity grows thicker, and in the end she becomes a villian and breaks Seifer out of prison and Seifer becomes her knight instead of Squall.

Squall has to do the thing, he promised not to do, and kill Rinoa.

However Seifer ends up saving Rinoa from her insanity and Rinoa is cured, but it's too late and Rinoa while dying shares her last bittersweet moments together with Squall.

idk that sounds so dark and depressing.