Planing a return

Hmm... okay, I'll give it a go and give you a thorough plot outline of how I imagine a Thamasa/Vector split would play out.  I like the principles PowerPanda applied in ThreeScenarios: each branch should have approximately the same length, should round out at three playable characters in the party, and should give each character an opportunity for growth in the story (Sabin and Edgar trying to piece their brotherhood together after estrangement, Terra and Banon forced to see things from the other's perspective, and Shadow demonstrating his skills as an assassin and alluding to his fears of authority).

For me to achieve the same result with a Thamasa re-pathing, I need to first inform you of my character interpretations/reimaginings.  Rather, it's how I read and rewrote Terra, Celes, and Shadow.  These could be considered spoilers for ReCast, so I'll put them in spoiler brackets.

Terra

Spoiler (Click to View)

I wrote an expanded backstory for Terra Branford, which she recounts at the top of the Zozo building after the party returns from the Magitek Research facility.  Having been taken by Gestahl, she was handed off as a toddler to Doctor Cid Branford (yeah, I know his canon name is Marseilles or something, this worked better).  He raised her in captivity in one of the magic-draining tubes, which he perfected with her input.  She was eager to impress him because he was the only person she ever got to interact with and his praise was the only positive feedback she ever received.  He basically raised her like an attack dog (that's why she can use every weapon type in the game... at least in my hack).  When she learned what the research was actually being used for, she set out to burn it and anything that had come from it.  She got 54 suits, troops included, before she was subdued and slave-crowned.
Augmentation was also perfected based on Cid's examination of Terra.  For this reason, Celes is younger than Terra.
The takeaway is that Terra is a biological weapon, and afraid she can't be anything else.  She's less a robotic 'what is love?' character and more a tormented 'will I ever be more than a weapon?' character.

Celes

Spoiler (Click to View)

I didn't buy the helpless damsel role that the game tried to cast on Celes.  She's an imperial general, not some love-starved twit!  More importantly, we know that she's applied ruthless tactics in the name of the Gesthalian Empire.  I recast her as a cold masochist, partly as an explanation for how Runic works and partly as a means of expressing the contrast between Kefka (the devil on her shoulder) and Leo (same spot, but the angel).  It also makes a better parallel to the Warring Triad, with Celes filling the role of Poltergeist.
Why a masochist?  Well, I imagine that when struck with a spell, Celes still feels the pain, even though her blade absorbs the magic as energy to refuel her.  As a coping mechanism for this, she taught herself not to feel anything at all.  She was also raised in captivity by Doctor Cid Branford, and the guy was a weapons developer... he's more about making tools than rearing functional human beings.  I can't go into more detail about this without spoiling another character from ReCast (and a very moving deathbed confession), but let's just say Cid's had lots of tries and he hasn't gotten one right.
Recall that Celes torched Maranda?  Cyan reveals this in Narshe when everyone meets after the first split.  This will be of particular importance.

Shadow

Spoiler (Click to View)

I use Clyde Arrowny instead of 'the Shadow' in my hack (he thought Baram's idea was corny.  He's also more sympathetic than the train robber of the original game; he's been sending money to Thamasa in secret from all of his jobs, to Strago, for the purpose of taking care of the village and his daughter.  This arrangement is a secret, so when Strago sees Clyde and alludes to it there are harsh glares to be had.
Shadow has a long talk with Terra about his daughter on the boat ride to Thamasa, dovetailing the exchange with Strago later.  This interpretation REQUIRES Clyde be present to save his daughter from the burning house, but also requires he seek out the rampaging espers to protect her from their destruction.  Hence, he is included in both branches.

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I was thinking about it and the best place to set up a three-party split is right after Relm spills the beans about the Village of the Magi.  To get everyone that needs to be there in the right position to make this work, we have to adjust the intro to Thamasa.

Party: Leo, Terra, Shadow (Locke and Celes in tow, unplayable)
Rather than having Leo and Celes storm off, all five characters and their Vector entourage enter Thamasa together and request an audience with Mayor Gungho (whose house is changed to the biggest house in the town.)

New Asset: Big Thamasa House (map), Unburnt
Recreate the general floorplan with the Towns Interior tiles.  Have several chests, some locked, some not.  When one party returns to the burning house later, all locked chests are unlocked and all previously unlocked chests are empty (Siegfried stole them if you didn't get them in time!  ...Hmm... come to think of it, why didn't they use Siegfried in Narshe instead of Lone Wolf? Oh, I'm getting distracted)

Celes gives Gungho a gift from Emperor Gestahl as a sign of the entourage's good intentions.  Strago enters to talk to Gungho (triggering his naming sequence), but is startled by Shadow's presence and thanks him for the last payment.  Shadow glares and leaves.  Both Strago and Gungho deny the existence of magic.  Leo expresses the importance of finding the Espers, but to deaf ears.  Instead, Strago asks for details about the gift.  Celes glances at Locke and leaves abruptly.  Locke chases after her.  Leo and Terra decide that there's something suspicious going on and set out to investigate the village (and see all the little scenes of magic).  They can talk to people about this, but the only one who'll spill the beans is Relm.  Shadow comes in with Celes and Locke, who are still arguing (or rather, Locke is pleading and Celes is cold-shouldering him).
When Strago denies Relm's claims, Celes casts an ice spell at him, which he deflects.  With no other option, he reveals the secret of Thamasa [the scene that usually happens after the fire].  Strago agrees to escort Leo and Terra to the holy ground in the mountains, while Celes and Locke stay in Thamasa to keep an eye on the Magi (and the Imperial troops).
This is where the split begins, cut to the black screen and pick a scenario, kupo!

Locke, Celes, Relm
At the Thamasa Inn, Locke continues to plead with Celes to talk to him.  She asks why he thinks she's a good person, he knows nothing about her.  He doesn't have an answer.  Then Celes explains how she took Maranda: she set the village on fire and promised to put it out with her magic only when they had surrendered unconditionally.
Then there's a loud rumble.  The package Celes gave to Gungho had exploded [yep, it's a Cecil reference ], setting off Thamasa's supply of enchanted weapons.  Relm runs into the inn begging for help; her friend lives in that house and he's still inside.  Relm runs out and starts fighting the fire, to the harsh wrath of Gungho who proclaims magic is forbidden (they don't know that the imperials already know).  Celes joins in to try to extinguish the flames, declaring this will not be another Maranda.  The villagers join in too, but the inferno is too intense; Celes volunteers to go in and find the survivors, with Relm insisting she go too.  Celes declines the offer, but Locke points out the girl clearly has magic too (works better if Relm learns spells by leveling) and could be useful.  The three charge into the house, playing out the burning building scenario as it would have in Vanilla, with the exception that the unconscious child behind Flame Eater is the little pyromaniac boy (still works, smoke inhalation wipes the party and Shadow still shows up to rescue Relm... and the others simply because they're there).

No sketch tutorial here; Relm will briefly explain how she wants to capture the life of the flames in her paintings, with the players working out its utility through experimentation.  Note, I generally change the sketch results to be more useful given the situation (casting ice against fire monsters, etc.)

Terra, Leo, Strago
Strago leads Terra and General Leo into the Crescent Mountains to search for the Espers holy ground, where they erected their monument to the Warring Triad.  No major changes here, except the Ultros Battle doesn't have a sketch tutorial in it: instead Shadow joins the party and helps push past the struggle with his Throw special (well, for me it would be Payday, he bribes Ultros to leave, but I recommend still converting it to a real boss fight).
When the espers approach threateningly, Leo and Shadow protect Strago (instead of the vanilla staging, Locke and Strago protecting Relm).  The events are otherwise identical.

Edgar, Sabin, Cyan
I'm... not ready to plot out this branch yet.  Unlike the other two, it'd have to be from scratch, be relatively short (15-20 minutes of playtime ending in a boss fight), and explain how the rest of the Returners escaped Gestahl's vector trap.  I _do_ know that it involves the Tea lady, at least!  Perhaps Cid too.
But probably not these three.
We should collaborate on this one.
Rough Idea: everybody is taking tea in the Imperial Palace barracks. Right as Edgar is about to take a sip, Cyan stops him and notes that the drink looks odd. Recognizing it as the same poison that was used at Doma, Cyan slaps the cup aside and grabs the tea girl, demanding an explanation. Edgar insists that approach is too harsh for a lady, and instead sits the tea girl down for a chat. Turns out Leo was sent on the goodwill mission to get all the Leo loyalists out of the city, so that Gestahl could let Kefka out. They're going to execute the Returners, capture the Espers, and then use that power to force the door to the Realm of Phantom Beasts and fetch the Triad.
Sabin sends Gau (and Mog if present) away to warn Setzer and get the Airship, since they can climb down from the palace safely. Then Edgar suggests they might be able to escape themselves by climbing down the broken cranes from Kefka's previous air assault. They fight their way past magitek soldiers and IAF pilots to the top of the palace.
...Seems too short, though, and I can't think of a good boss fight. Kefka would be anticlimactic because he'd just have to run away. Gestahl might be interesting (being he's a VERY powerful magician), though he honestly might be too powerful for these three to wound him (and thereby win their escape).