Planing a return

Well, if you're looking for ideas on scenarios for expansion, I'd first say start with PowerPanda's work. 3 Scenarios does a wonderful job balancing the party split, by giving Terra more to do before the meeting of Narshe [half of Sabin's scenario] and ensuring each group introduces only one new character. The story flows much more smoothly with those changes.

Other places that could benefit from expansion are the Zozo conversation with Ramuh (which could really benefit from being different for EVERY character that's there to contribute, rather than a bunch of rephrased lines with different speaker attribution). Locke being there could send the conversation down the myths of the War of the Magi (his grandmother's stories). Celes could turn it to talks about esper augmentation. Edgar to Vector politics and the fate of his parents. So on and so forth.

Likewise, the flashback with Maduin could benefit from a greatly expanded dialogue and event script. It could even extend into another scenario about Terra's childhood within the Magitek Research facility (I thought it was odd that she remembered being a baby, but not the rest of her life; my own hack addresses this in dialogue).

As far as side quests... there's a lot you can do there too. Umaro's recruitment was a bit of an afterthought and could benefit from a side quest... perhaps a scavenger hunt in the ruins of Narshe to gather components so he can finish carving his idol. Sabin running into his master Duncan felt pretty poorly executed: finding Duncan could have involved tracking Vargas across the World of Ruin (and fighting him several times). Edgar doesn't get a power up in the World of Ruin or have plot beyond his re-recruitment: maybe you could restrict the Debilitator and have him invent it by following a series of Figaro scholar notes to the library in the Ancient Castle beneath it (tying it to the Odin esper). Relm also doesn't have a significant quest after recruitment; the fake mustache is just shoved in a chest somewhere.
You could move Locke's World of Ruin recruitment to Narshe and make the Phoenix Cave his side quest... put the Thief's Knife and the relic that turns Steal into Capture in there. Mog and Gau are also lacking significant power-ups in the World of Ruin (as is Terra, though she does get a major one on recruitment...) which could be tied to side quests.

Hopefully these are helpful ideas. Basically, look at which characters got neglected in the second half of the game and expand on them.


EDIT: I had an idea for Mog. You could track how many dances he's learned (and tie at least one of them to a World of Ruin Zone, possibly the Water Rondo), and then have him talk to Draco at the Opera House. If he's learned all eight, Draco teaches Mog some new performance moves and gives him a "Stage Actor's Guild" card... a relic that when equipped keeps the 'Dance' status from being set and prevents 'Stumble'. Thus, Mog could choose a new dance each turn, or do something else after performing a dance move.
As for Relm... well, World of Ruin Albrook is pretty unused. Maybe there could be an event in the tavern there where Relm agrees to boost business by drawing caricatures, and you have to run around town convincing people to go get their portrait drawn. Once you get enough patrons, the bartender could give you the Fake Mustache.
Gau's already got the task to impress his father, but that can be expanded with some kind of fetch-quest. Basically a bunch of short airship trips to various stores to trigger scenes like the clothes shopping and the dining etiquette. At the end of it all, Gau could get a relic that keeps the Rage flag from being set (really powerful, actually, because then you can stack monster flags on him).

OH! One more thing! If you manage to save Cid, he tells Celes to bring Locke to meet him. Yet, if you do in the vanilla game, nothing happens! That scene could use an overhaul.