Before Things Go to Hell..

[Clo]

Are there any sidequests that are time sensitive, that I can't do once the World of Balance goes under? I'm trying to determine this through walkthroughs and failing.

Before I go to the Sealed Cave. Hell, I haven't even gotten Relm yet. But I don't want to miss anything.

EDIT: I'm playing the PSX anthology version, so it would be SNES.

[Jowy]

Depends on the version of the game you're playing. Water Rondo is unobtainable in the World of Ruin unless you're playing FFVI Advance (assuming you picked up Mog that is). Golem and ZoneSeek magicite from the Jidoor auction house can be purchased for a total of 30,000 GP...don't think you can pick those up later on. Seraphim magicite is obtainable in Tzen off to the side in some trees. You can wait on that however and get it for a whopping 10 GP once the World of Ruin hits but it is an exponentially faster way of learning Cure 2. You can only fight Intagnir on Triangle Island in the World of Balance...he's hard as smurf though so I wouldn't recommend it unless you want a full bestiary/rage list. Mailing the letters for the wounded soldier in Mobliz to get your Tintinabar is another. Make sure to have three Mystery Veils, and a Bard's Hat or two in case you feel like building an "unhittable" character or four. Chainsaw from the clock in Zozo.

And don't forget the Atma Weapon in the Sealed Cave!

[espritduo]

You can still get the Golem and ZoneSeek magicite, as well as the Chainsaw in the WoR. The primary things to do in the WoB are fighting all the missable enemies (to fill out the bestiary if you're playing the GBA version, or just to get them on the Veldt for Gau to learn their Rages in general) and getting a few items that you don't want to miss. The Atma/Ultima Weapon in the Sealed Cave is the biggest thing you do not want to miss. The often-missed enemies are the Intangir(Triangle Island in the north-east corner of the map) and the Grenade(the large forest to the west of the Veldt). Other little things are the Hero Ring in the old man's house north of Kohlingen, since that house is not in the WoR, and the stuff in the Imperial Base that gets unlocked after the peace talks with Gestahl.

[Clo]

I updated the OP to name the version I'm playing -- I completely forgot to mention.

[Wolf Kanno]

Then Water Rondo is the biggest thing you can miss. Grab Mog and return to the cave on the Veldt to jump into the Serpent Trench to learn the Dance. Afterwards, you'll have to walk back to your airship.

Most of the missable items in the WoB are usually hard to find items in one time dungeons like the Atma Weapon in the Sealed Cave or the Break Blade in the Magitek Factory. There are two missable scenes you haven't reached yet. One is after the dinner with Gestahl, if you truck back to Maranda where the Blackjack is stationed and walk into the engine room, you can see a scene with Setzer and Cid that is very easy to miss and gives some foreshadowing for later in the game. After you leave Thamasa, if you come back with both Relm and Strago in your party, you'll get a minor dialogue event that introduces Gung-Ho and foreshadows the Hidon event later.

The conversation with Emperor Gestahl itself will bork you out of getting a few items if you say the wrong thing. There is also the Wounded Soldier quest in Mobliz. He asks you to send items to his girlfriend in Maranda and completing it will net you an easy Tintinibar.

[Jowy]

and the stuff in the Imperial Base that gets unlocked after the peace talks with Gestahl.

Good reminder! There are also a few treasures in Doma Castle to pick up as well.

[Sephiroth]

Most of the missable items in the WoB are usually hard to find items in one time dungeons like the Atma Weapon in the Sealed Cave or the Break Blade in the Magitek Factory.

The Break Blade can still be stolen, so even if you miss the treasure chest in the MagiTek Factory you can get it.

[Wolf Kanno]

Yes it can be stolen from the Outsiders or won in the Coliseum but by that point its not really practical anymore and its more of a saving your semi-perfect game.

[Jowy]

bump!

how far along are you guys now?

[Clo]

He just got Edgar in the World of Ruin... but I insisted he play Lunar 2. And then that mayhem happened.

[Jowy]

That's a sign to finish FFVI first then!