Restricted Cast, who would you pick?

I wasn't really thinking character plot rewrites... more along the lines of downplaying their roles in the game and making them non-combat characters.

For instance, Edgar is the king of Figaro.  He's got an entire country to lead; he can't be running around in caves with some witch.  Setzer can pilot and maintain an airship without going into battle.  That sort of thing.

Your breakdown of action commands is really good, though.  I particularly like what you described for Sabin.

In that case...

Sabin - teaches Blitzes in exchange for money or quests, has his own gym/monastary/dojo. Quests related to lawsuits and notices to appear in court accidentally sent to him instead of his twin brother Edgar, his rival Vargas and Vargas' school/dojo. Have some quests related to very rare stat boost items (steroids, growth hormones, and related stuff) being delivered from Mog. Some quests about Duncan and Duncan's wife.

Vargas - same as Sabin but evil, presumably pro-Imperial. Include some quests related to Duncan.

Strago - manages espers, magicite, does magic training for the party. At least I assume he can; given that you can learn magic from both items and espers, it stands to reason that a magically gifted human could teach you some spells too (after appropriate side quests for "ingredients" like Brachiosaur bones that are actually event flags). Would be nice to have a less grindey alternative to esper and item learning, and more content in optional areas to explore. Add some quests related to Shadow and Relm.

Relm - changes the party's costumes, give you sidequests for item crafting where you find rare art materials (sets event flags, not actual items) and she paints useful items into reality for you. Sidequests to unlock menu decorations and recolors, and make furniture for your base/house/faction. Add sidequests related to her mom and dad, with options to deliver info to Shadow and lie to her, or to lead her to Shadow and have him very reluctantly play the father while sweating about the money he owes Child Services. Her mom may not actually be dead, just doing something shameful far away or partying it up with drug dealers and using her magic to get high.

Gau - combat training, he takes the form of any monster you've already fought so you can learn blue magic or monster skills from them. Can't steal/morph items, or get item drops though. He would call it "dancing", of course. I can't think of any way to make Gau's story any worse/sad, which is really saying something.

Mog - uses mysterious moogle connections to transport you from one town/cave/forest to another across the world. Maybe add giant mushroom themes and jokes about what exactly the verdant green plants in the moogle gardens are, to which he always forgets how to speak to humans and defaults to "Kupo?" before changing the subject. Teaches you various dances or how to "jump" in exchange for certain seeds and "herbs", or delivering a package through port security to seedy strangers or other moogles without opening the package.

Umaro - mining minigame, or gives you quests where you trade carvings for random stuff he found, made, or took off the bodies of adventurers and imperials he killed and presumably ate. Help him finish a drum set, xylophone/piano/lyre, and guitar made from parts of various critters, and you unlock the music/sound test and a no-shit in-game music creator with a few music slots you can play from the menu.

Gogo - name change, let's you switch out skills like GP Rain, Throw, X-Magic, X-Fight, and so on after you complete various quests. Obviously can only do this for one character at a time using event flags to prevent you from giving everyone X-Fight. Can occasionally be found wandering around your base disguised as a character already in your party or your lead character. Maybe make the quests hits on clowns, in a mime vs clown running gag, or steal stuff necessary for Gogo to transform into various people.

Setzer - airship pilot, casino (minigame) owner, and presumably convicted sex offender (sure likes to kidnap women), who offers quests related to all these things. Maybe get blackmail on a politician in different areas to get pardons or records destroyed, or just destroy the records yourself.

Edgar - can offer you loans at interest since he's a head of state and has his own national bank, makes mechanical contraptions for you if you do quests for him. Gives quests about getting other countries to take his loans or pay them back, usually involving sabotage or blackmail of leaders. Deliver Edgar's hilarious and awful love letters to various female NPCs and try to escape without being arrested or beaten by their male relatives, boyfriends, husbands, and the police.

Locke - offers quests, steals stuff from targets for you in exchange for payment or quests, maybe runs a thieves treasure hunters guild. Include a quest to steal "personal effects" from Celes, to lampshade that Locke is either a simp, creep, or utterly clueless about women in vanilla. Quests about Rachel too, and the old man who has her in stasis, her family (who may not actually be dead), and what actually happened in all this vs Locke as unreliable narrator.

Shadow - offers assassinations, gives quests, loans you his dog after you find it for him. Have him offer quests to destroy records about how much child support he owes Strago.

Celes - as a general, she offers military, sabotage, and spying related quests and minigames for the player, and I assume teaches magic and maybe Runic too. Could be you fighting against Leo or Kefka for different effects in the minigames and quests. Include a quest to deliver a restraining order to Locke, Edgar, and Setzer.

Leo/Kefka/Gestahl - offers the same quests as above, but for the Empire. The quests match the nature of the character - Leo is honorable, Kefka is insane and evil, Gestahl is rather clueless but also power hungry and conniving. Some of Kefka's quests are hits on mimes. Mimes should be obsessive and compulsive, clowns omnicidal narcissists.

Cid - offers esper/magicite/magitech/magic quests for the Empire. Has an awkward or strained relationship with Celes. Have quests related to her mother.

Bannon - Pretty much the same as Gestahl, just for the Returners.

The old guy who meets Terra after she meets the Esper - Returner and Narshe quests.

The Narshe Mayor - Narshe quests.

Terra - NPC that offers quests related to her lost memories, espers, her mother's family here in the non-esper world, teaches some magic in exchange for that or money. We have no idea who she is or where she comes from, would be nice to see that. Some jokes from both her mother and father's family where they talk crap about humans and espers, and maybe some fights between the two families to Terra's eternal embarrassment. Maybe rescue her mom and dad too, don't let them die, and give her brothers and sisters. Would be funny if one of her siblings was literally a dog esper, and the embarrassment and shame that would cause her mother's side of the family... and of course furry jokes.

Cyan - teaches you sword techs as you finish quests for him, related to: Doma, burying his wife and child and other people of Doma, putting their spirits to rest and removing his bad dreams (by going in his head and purging that demon whose name I forgot), finding a powerful sentient sword, returning his sword to his ancestral tomb/shrine. Maybe add jokes about his kid not actually being his own, with the option to show or not show him the letters his wife wrote to that effect and make the kid's sprite look nothing like him.

These are just ideas, don't need to use them for all characters. Some of the quests should be repeatable, which I assume you can do by resetting some event flags when a quest is complete or some time afterwards.

And yes, many of these are horrifying and full of fridge horror implications.