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-The short is set 8 years after the end of the second movie, Sheryl has not awakened yet and Alto is still missing (I've read conflicting reports on the actual time so it may not be 8 years but it is some time after the second movie)
-Ranka is on a Interstellar tour
-New Protoculture ruins are discovered called "The Well of Souls" and Ranka goes there
-The ruins react to Ranka's song and she feels Alto's presence and jumps into the ruins
-Inside the ruins show many memories of Ranka (apparently all the memories are new scenes)
-Sheryl appears there although not physically obviously (Probably thanks to her earring?)
-Both sing in duet a new song that supposedly is really good
-After the song ends a beam of fold energy shoots and shows Ranka the exact location of Alto

So in summary:
https://c.tenor.com/UY2s30zUK3sAAAAC/nothing-wow.gif

Joking aside, the people that have watched it say that the new songs are really good and the animation is excellent, also all the characters appear, Mikhail, Klan, Ozma, Luca, Nanase and even Brera (just in the memories though).

But yeah it really didn't add much new and just maintained the Status Quo instead of advancing the story even if a little bit, personally I still prefer the new end credits to the Blu-ray box version where it heavily implies that the end credits concert is after Sheryl woke up and that the blue shooting star is Alto coming back.
In the end more Frontier is always welcomed but come on just let Alto and Sheryl reunite already!

that epilogue you're talking about also didn't set a time or space; it also didn't clarify if it's real or not. So, you banked a lot on an additional concert on a special second release set as proof she woke up within 8 or more years. Comas don't work like that. In fact, it only proves that with all the technology developments in the Macross universe, they still have yet to uncover if there's a solution to waking someone from a comatose state.

Also, on Japanese Twitter, a spoiler said Ranka had still (I repeat, still) been singing to Sheryl to try and wake her up despite the timeskip. Talk about being dedicated to your friends.

Meanwhile, I don't really know what anyone expected of this short film...?
Everyone continued asking for "more of Frontier" once the show ended and now that it's got a short film which ruins their vision of the ending of the movies, it's problematic? Especially since, in interviews, Kawamori kind of hinted at this viewpoint when he called Sheryl's waking up and Alto's return as "eventual." Meaning, there was no set time even during the epilogue.

Regardless, neither TV series nor movies are more canon than the other since both count. So if one version doesn't stick, there's always the TV series ending even if there's no canonical romantic conclusion.

First, the confirmed stuff: Freyja dies by sacrificing herself to save everyone via singing, supposedly has a new song. Think of Ranka's "Afterschool Overflow" song from Sayonara no Tsubasa, because Japanese Twitter calls it "her love confession" song...

Next up is unconfirmed. However, it's hard to read Japanese when a poster is using the equivalent of "broken English" in their posts. So all of this is based on various tweets on Twitter plus maybe two reviews I found online: Freyja maybe leaves Hayate a child? So they're likely... already a couple(?). It's unclear if the child is hers or not. However, a bunch of fans of the ship online have been extremely happy despite... you know, the depressing conclusion of Freyja herself. So... maybe it's valid. However, I have yet to run across any confirmation of how this comes about so I guess it's extremely spoiler heavy and deeply hidden if it's true or something got lost in translation. Regardless, if it's like someone on Macross World forums said and it's due to her runes... oh boy, Jesus Ranka strikes again. Kawamori really loves that. For now though, it's unconfirmed!

They already gave the fans hints about Sheryl's recovery and Alto's return, twice actually with the original end of the movie and the new end credits from the blu-ray to make it clearer. Then they announce a new short film hyping it as the first new Frontier content in 10 years just for it to be well... more hints about the thing you already gave us hints about. :heh:

It's not that Freyja leaves a child, what happens is that the Bad guys have the technology to create new Star Singers so they make one that eventually evolves into Yami-Q-ray (the evil versions of Walkure) and in the final battle they start this process again but this time Freyja sings until she dies and her song had an influence in the process so the newly born/created baby star singer develops a rune like Freyja's, and in the end Hayate decides to raise this new star singer in Windermere, so it's not really a biological child of Freyja or Hayate but it seems that the kid takes a resemblance to Freyja because of the influence of her song.