Those are some awesome ideas, I've actually made similar assertions in the past in regards to how a proper epilogue could work although I've recently come to the conclusion that it would've probably worked best as a non-integrated DLC. Not that I wouldn't want it included however, as I would, but that thought was more of a compromise... Firstly, it maintains Tabata's original vision allowing players to begin their journey in a relatively short period of time, and secondly it would allow players doing subsequent runs too opt out of the epilogue if they pleased. That seems to me like it would've been a fair way for every ones wants to conjunct while at the same time giving this game its much needed Insomnia epilogue...
Now, as for what the story content could've consisted of I like that idea you mentioned that takes place before chapter 1. Personally, I've always thought they could seamlessly utilize the script from parting ways and deliver that experience too us in-game, with a bit extra on the side of course
- such as spending a bit of time with Regis (perhaps the day before?), encountering unique NPC's in the streets and growing some attachment to the citizens of the capital in the process, sitting down and enjoying a meal at the restaurants, and just enjoying the metropolitan marvel that is Insomnia... this place holds so much significance to the world, the main characters, the plot.. it's what ultimate end-goal we're slowly but surely fighting towards the entire game and the destruction as we witnesses it as we return is supposed to have a rea significant impact on the player but because we don't even spend 1 minute their establishing any sort of connection or attachment too it the feeling just doesn't resonate through the party back too the player, not as strongly as it should anyway... and I think that's a real travesty. So many players would connect more with the end-game, and this game would be appreciated much more significantly than it is now if this had been a thing, but unfortunately it never will...
If you think that's an acceptable compromise than you're missing the point... it's not just about having an epilogue or a prologue, it's about having one in a playable Insomnia. Who would we fight? Again, you're missing the point... although, I mean there are plenty of potential training partners like: Nyx, Regis & Cor but than again it's not about the fighting and FF has never, ever only been about the fight. It's about the story for me, at least here in this instance... It's about bringing the main city into the main game and actually making the player feel attached too it before spending 40-60 hours trying to reclaim its throne.