Probably not something I’ll ever do anything with (because I’m bad at project management and this feels like a long piece) but I wanted to get it out there:
Using their Diving Pulse powers, Byleth keeps reliving their time at Garreg Mach and into the subsequent war as they try desperately to either stop the war or save everyone. After four cycles of fruitless attempts to avert catastrophe (one for each route lol), Sothis gets an idea. Clearly war was in motion long before the fateful meeting in Remire (the farthest back Byleth can push their Divine Pulse), and thus nothing they can do will stop it. However, Sothis has been awake for a while at this point (even if she keeps fusing and then being re-separated from Byleth) and feels like she can go back further than Byleth. So, she gives Byleth a kiss on the forehead and tells them to let her fix it for them, and proceeds to turn back time over twenty years.
What Sothis doesn’t account for is she’s still bound to her heart/crest stone, and so a wrench is thrown into her plans when she arrives in the past to find herself staring at a surprised Sitri. What commences is Sothis and Sitri’s Roadtrip To Save Fodlan, ft. Jeralt (who is a little freaked out about kidnapping Sitri from the monastery for some divine mission they can’t tell Rhea about because he still trusts Rhea at this point, but he’s weak to Sitri’s will) (Addendum: Sothis came dangerously close to fading completely in the CF run so her memories are a little spotty for the end of that one, but she Does remember Rhea going batshit, and she also remembers Rhea treating Byleth especially weirdly in the church route and thinks it’s probably wise to not let her daughter know she’s awake and aware just yet. It does not occur to her that Rhea will freak out just as bad to realize Sothis’ crest stone Sitri has gone missing but ¯\_(ツ)_/¯)
Although somewhat hampered by Sitri’s weak constitution (bolstered by Sothis but not fixed completely) and later Knights of Seiros frantically looking for them, they and any allies they manage to make (I imagine at least a young Alois shows up and instead of turning them in he’s like, “Captain! Lady Sitri! You forgot to tell me we were going on a trip! =DDD” bc sure he’s a Knight but he’s Jeralt’s SIC first. Maybe Aelfric too since he hasn’t been twisted by his grief yet. Probably at least one person from the Empire, Alliance and Kingdom. I’m nominating Tiana von Riegan bc Sitri deserves female friends and I feel like Claude got his obsessive curiously and devil-may-care attitude from her so obviously she would be down to investigate the dark secrets of Fodlan. Not sure about the Kingdom or Empire representative. Lambert would already be king at this point and Gilbert/Gustav and Rodrigue are too loyal to go off on a boondoggle. Maybe Rufus, who isn’t acting as regent yet and thus has no obligations, and also since he’s a blank slate of a character except for his apparent womanizing and I like to imagine he’s what Sylvain would be like he was depressed about not having a crest instead of having one. Or maybe Cornelia: she hasn’t been bodysnatched yet and was apparently a lovely person before TWSITD replaced her. As for the Empire, Patricia would be a little young at this point. I’m guessing she’d still be an academy student or even younger since that was when she met Ionius and I’m not sure how long after that she had Edelgard but I imagine it was less than six to seven years. Ionius himself is out for the same reason Lambert is and… frankly all of the named Empire nobles we have aside from them are terrible people. Maaaaaybe a pre-bodysnatched Arundel? I suppose Hanneman and Manuela would both still be associated with the Empire at this point, too, although I imagine Manuela would just be starting her Opera career. Maybe they can bribe Hanneman with research on Sitri’s Crest of Flames (which she has because why wouldn’t she with Sothis there?).)
Anyway I’m a little vague on the middle parts but suffice to say Sothis accomplishes her goal to put down TWSITD and clean up any loose ends that would come back to bite Byleth twenty-odd years in the future. And at some point has a long-overdue talk with Rhea. ‘Cause what the hell, Rhea? But, in that time she’s become incredibly attached to Sitri, and doesn’t want her to die giving birth to Byleth. So, Sothis chooses to make one last sacrifice for Byleth’s happiness and does what she has four times before merges with Sitri (and I don’t care if this is impossible because it’s my AU and I get to make the rules). In the epilogue, Byleth is born safe and healthy and alive, and most importantly unburdened by the fate and responsibilities that weighed them down the first time (first four times?) around. Also I like to imagine Byleth maybe can’t see Sothis, but they can still hear her. And because I want Byleth to still wield the SotC, maybe they don’t have her crest stone in their chest, but the working heart they got thanks to Sothis’ interference is enough to at least get SotC up to its regular form, if not Sublime.
But anyway the fic ends with Byleth meeting the three House Leaders not as a professor, but as a student who isn’t affiliated with any of the nations and thus gets to choose which house to join. The fic ends before Byleth makes their choice, but with the implications that regardless of what choice they make, they have a better, more peaceful future waiting for them.