Planing a return

I'd like to see a second (maybe a third? ) scenerio split. The whens are easy, but the whats and hows are killers.

Hmm... I can think of two good spots that could be rewritten for splits.

The first is the Thamasa campaign.
If they meet Strago at the dock when they make land, they can fork into two parties with Leo and Celes (and Shadow) going into Thamasa (culminating in the burning building with Relm), while Locke and Terra (and Strago) get chased by Ultros in the mountains.  It pushes back Relm's introduction as a playable character (only becoming available right before the floating continent) but adds more non-inn development for Shadow.
The third branch could be Edgar and company back at Vector... an enemy gauntlet as they make their escape (with fewer enemies the nicer you are to the tea girl).
The only problem there is Mog... either Mog needs to become a mandatory character so that he and Cyan can have some adventures in a fourth scenario, or you'll need to be able to select party members in Edgar's branch.

The second is the World of Ruin post-Lonely isle.
Again, you have the Celes branch which plays like Vanilla through Figaro (Tzen mandatory).
For option two, you have a Locke branch starting in Maranda (with the wounded carrier pigeon), followed by a stopover in Jidoor (w/Starlet, meet up with Relm), then leading through Zozo (to get Cyan) and finally to Kohlingen.
Lastly, option three would be a Terra branch, covering the Mobliz events [w/destroyed Mobliz connecting to Cave on the Veldt and Gau], a run-in with the Zone Eater, and then a half-esper flight to Kohlingen (or a devil's road situation where the Zone Eater's 'exit' is the Coliseum... hmm, so THAT's where he gets the monsters!).
All three branches wrap up with a reunion with Setzer to go dig up the Falcon.
Of course, that kinda eats up most of the premises for side quests, so maybe that's not such a great idea.

For the events at Thamasa, you would need to do some re-routing. The parts to keep in mind are:
1. Relm and Strago need to be in Thamasa in order for the events of that town to play out correctly. Taking either of them out will have drastic effects on the storytelling.
2. Strago also needs to be in the Esper's Cave to explain the 3 Statues. You could potentially get away with having Leo or Celes explain them, but not Locke or Terra.
3. The Ultros fight is set to only end with Relm sketching Ultros, so you will have to update that script and drop the HP if Relm isn't there. You would also need to explain why other party members have such a visceral response to Relm offering to paint Sabin's portrait.
3. Terra needs to be in the Esper's Cave for the scene with Yura to work.
4. From a gameplay perspective, the Thamasa events are meant to bring Terra up to speed on her skills, and also allow you to get skills started for Strago and Relm. The inclusion of Locke was necessary from a story standpoint, but it doesn't work that well for gameplay since he's also required for the Vector run. In Divergent Paths, I've switched out Locke for Leo.

I love the idea of an Edgar scenario though. That would include Edgar, Sabin, and Cyan for sure. From a gameplay perspective, you'd probably want to include Setzer, as he needs some time to build up his skills. However, he's also the easiest absence to explain, as he and Cid are fixing up the Blackjack. I would NOT recommend using Gau or Mog for this, as every event that includes them would have to include a check for if they're not there. That was extremely difficult for me when I added Shadow to Locke's scenario in Divergent Paths. It could be handwaved though with a single scene, where Edgar explains he's going undercover, and Gau and Mog would stand out too much.

For the World of Ruin, you either need to make ALL of the character recruitment part of branching scenarios, or you need to make NONE of them part of scenarios. However, if you could figure out a way to make Terra mandatory, that would fix the issue where she just shows up in the ending cutscene if you don't already have her.