I looked online, and there seems to be some contradiction on what are "Must do things" before you enter the World of Ruin. (I.e. Once you enter the World of Ruin, you can't get a certain spell, item, dance, etc.)
Here is what I know so far.
- Mog - Almost impossible to obtain Water Sonata dance in the World of Ruin, unless you randomly earn it during the Leviathan fight.
- Mog - Must rescue Mog in World of Light, or he doesn't show up in the World of Ruin?
- Strago - Lores? Any that you cannot get in World of Ruin?
- Gau - Rages. Someone said the Ultra-Powerful "Stray Cat" cannot be obtained in the World of Ruin. Any others?
- Shadow - Must wait for Shadow on Floating Continent, or you can never get him back again for the rest of the game.
- Items - Magicite at Jidoor auction??? (Can't confirm or deny)
- Items - Magicite at Tzen? (Can't Confirm or deny)
Anyone confirm or deny any of these? What else does everyone know? I will add to this list as we go.