I also haven't played rebirth either, but for remake I liked the idea that the entire game was a xanatos gambit. Sephiroth, having lost at the end of 7 reaches back to cause Cloud and co to try and make a future where aerith doesn't die. Of course, in the original timeline, Sephirth dies, and the heroes win, that's undone by this as well. The whole time ghosts thing was weird, but I can see how it's narratively important to raise the stakes and have an opponent you're fighting against.
If any of that played out in rebirth, dunno.
As for my next game, from the dev side I wouldn't quite say ground work is laid down... I have purposefully not started work on it. I'm not the type to say odd notes and conceptualising counts as work, even if it's important for speedy development.
As for it having time loops? Fuck no! I'm dropping that like a hot potato. Something I didn't realise when I was first making this was that I was designing a maze, and that as corruption levels changed, the walls of the maze changed with it. The guides are your map of that maze and I've made them as clear as I can... But they're a lot of work and I don't like making them... What comes next will be a lot more linear.
As for completion percent of Cabin goes, 3 Characters are effectively complete, 3 characters are almost complete, and 3 have are a bit under half done. Overall it's probably closer than you think. And even when it is "done", development isn't going to just stop. While I spin up the next game I'm still going to be adding more missing scenes and building upon fun ideas in Cabin.
And yes, there will be more gratuitously large breasts, though with a little more variance in size. It was a complaint that all the breasts in Cabin were a little too similar and I plan to remedy that.