Someone has to die, that's for sure...
I liked Riku the best out of all the characters, so it would be cool to see him actually go evil on everyone and take out Kairi in jealousy and rage, (over her affection with Sora).
Then, you'd get an extremely hard fight against Riku and he'd tell you "I was always...better..." when you keyblade his @$$.Then, a CG Epilogue showing Sora suffering with powerful, epic music.
That, or something else...I always liked Riku better anyway...