Funnily enough I did kick around a similar concept where instead of future Cait going back in time, it was the Champ going back to when Cait was in the temple. Shenanigans ensue where we go on the run with Cait. I forgot exactly how I had planned for that to happen, I think it had something to do with Cait's royal ancestry getting the Sultana wanting her head. Maybe Tollus and the champ both fall into a time portal while fighting and Tollus is able to corrupt the Sultana? Either way Cait being away from the temple for the first time and seeing the outside world ends up with Cait falling for the champ and disavowing the temple, especially when she hears how it's going to be a gateway for the Demons to corrupt people.
Cait decides to reform her faith to be more focused on "pure" love. Inspired by an encounter with Mallach himself. I was sort of planning on having it be the catboy comes to the same realization as Cait does when the champ pops into the past and tells Cait about how shitty the future is, and encourages her. He's also not a fan at how things are being run. (Yes, this will involve retcons to the base game. But if it means no more grooming temple...) Leading to a whole questline that either ends with Cait failing and leaving her homeland in exile having totally cut ties with the temple or her succeding and becoming the new "prophet", leaving to spread the newly reformed Mallachite faith to the other temples.
How I was going to connect this to the larger game, especially since this involves timeline fuckery, I have no idea. I think I was considering some sort of new game plus mechanic? Obviously this was way more ambitious than it has any right to be, but I enjoy coming up with these expansion ideas. I had a much more doable expansion idea in mind focusing on the Kitsune that at least doesn't involve fucking with the timeline. I guess I wanted to have an expansion focus on each of the Seven, changing them to realize "Hey, we kind of suck and were going about thing wrong." and either submitting (Mallach, Keros, Lumia, Tira, Nareva) or allying the champ, giving you a bunch of cool power fantasies and effecting the main plot.
I recall Savin one time mentioning how he wanted a "battle" mechanic where large scale army fights could play out:
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and I was like, "Cool, that sounds useful", as having large scale wars would play a big role in at least two of these expansion (This one and the Kitsune one). Of course, holding my breath for this mechanic is fool's errand. It's almost certainly a never ever, or if it does get into the game- it's going to be so watered down and empty.