Flashback has them meeting an enemy that they might before...given that this is the Rebellion Milieu... I would think that along the way there are no other time travelling adventurers (at least, I hope not) it possibly represents an object or an idea...
Now, given even Ancient objects have been known to run down once their antimatter batteries are dead. And the fact that nobody since Secret of the Ancients really wants to touch the Ancients, save a few incursions by DGP & TNE - but they were not like the original trilogy which was o blurt out the fact: "It's the Droyne, Stupid!"
That leaves an idea...
For me, one of the things that would have repulsed early Terrans would have been the idea of slavery. Their critical interventions at various moments would have seen the rise and fall of many slave states. And, possibly being noble (not titular but character-wise) characters they would try to defeat this form of serfdom wherever or whenever they can. Now when they arrive in 1116...which of the major players actively ignored slavery despite adhering to the covenants of the 3I?
So begins the final battle to either take up arms against that force with some repugnant like maybe the Solomani Sphere, in the aims of abolishing something even more horrific...unless you subscribe to the Jackboot theory of the Solomani Sphere...
The first part of the adventure would be filled with thrills and many chances for saving the innocent. But, the last part will leave a moral quandry that could be resolved by advancing a reform wing within the Solomani Cause but at the same time threaten the existance of the 3I. Something that the adventurers, until that point had fought to create as an instrument of reform...
Anyway, that is my take on how Flashback could be made into an interesting module...if Marc would release all his background history notes of the various Milieus that the adventurers were to visit...it might be possible to get an adventure written.
So I do like your idea of it is to save humaniti but I would as you see, twist to try to save Humanti's soul, in the process create a Faustian pact that would consume the adventurers...
I tould you guys that I like things that are Dark...
