I went back and read all but the first 2 chapters over the past couple of weeks. Some moments struck me harder than they did on my first read through, such as Spitfire's death. Issues including Rarity not dwelling on Spike's demise for as long as she should have given their time together. Some of the details of battle can be hard to follow at times (especially the finale), but it does sort of give an impression of how chaotic it must be.
Another issue was how the pacing had slowed down halfway though and stayed that way towards the end, only to have the final moments condensed into less than a quarter of a sub-10k chapter. I knew it would be a tight squeeze even without the matter of Dash and Twi returning to their universe, but rushing over AJ's death and her giving virtually no reaction to her brother's fate felt so hollow to me, given how incredibly guilt-ridden she must have been. She doesn't have to live with the guilt,
but having the last of the main Apple family members wiped out like that was pretty cold.
One death I
was expecting from the beginning however was Captain Dash. Surprised it didn't happen at all, though part of me expect her to wield an element at some point (Honesty being the obvious choice, looking back). The fact that it was used against Trixie was all the more poetic.
With such a brief ending though, I can't help but wish for some sort of epilogue as a way wrapping up what was a somewhat underwhelming conclusion. At least the elements worked, but the cost was so heavy that I really don't think it should end so suddenly.
That said, the open-ended nature of this story does make me hope that the right person can pick up where BC left off. Though my headcanon is a little different than the one you posted:
Luna eventually lifts the sun after maintaining a star and moonlit night for a year since she was too weak to lower them. But not before Twilight discovers that Celestia's life energy never really left Equestia, and that it's up to her to find it while various factions who still see Luna as a threat try to stop them. Events lead to a much-weakened Celly returning and a race to get her to safety.
Ultimately, with data collected after months of research and discovering the right conditions to perform the spell that brought her there in the first place, Twilight finally feels that the universe is stable enough to warrant her return to Equestria Prime. Luna has help from her still-recovering sister to raise the sun for the first time on over two years. Twi and Dash proceed to leave, though the trials they've been put through have changed them to an extent.
The twist? Turns out the real Equestria went to **** in their absence. Hoping they were only gone for a few moments, they'd actually left chaos to envelop their world over a two year period. Roughly the same amount of time they'd spent in the alternate universe. Kinda like the 2001 Planet of the Apes. Oops.
And then it just ends completely. *
maniacal laughter*