I agree with the longest final boss stream, but I would probably switch Master Xehanort and Armored Master Xehanort so the un-armored would be the last, maybe it's because I love final boss fights that end with no upgrades, something like that final fight between Cloud and Sephiroth. No upgrades, no power-ups, alone in-battle, fight to the last breath, etc.
Shantotto, and Gilgamesh should play roles in KHIII.
What would a younger version of Lightning look like?
Probably less broody
Mains: Squall, Gabranth
Subs: Vaan, Laguna, Warrior of Light
Learning: Ultimecia, Lightning