Interstella 5555/WMG

Arpegius used to be a member of the planet's army

He's always doing the actual salute, while the rest of the band waves or looks at things respectfully.

Makes sense. He's the most competent in a fight and even seemed to fly Shep's ship for a bit at the end.

Shep and Stella were together before the events of the film

Shep isn't really an Ascended Fanboy. Rather he and Stella were a couple before the abduction happened. That would explain why, compared to the rest of their native planet, it seemed personal for him. Also, it would explain why Stella snapped into a Dream Sequence when she held his hand as he was dying, and why she was devastated when he died. Love At First Sight can obviously still be an option, but I think it's actually believable this way.

... I actually really love this WMG. Makes a lot of sense really, and makes Stella, the titular character, seem a lot less two-dimensional.
I'm not sure about that. In the Digital Love section he seems too happy for someone longing for an ex.
- How about thinking of them still being a couple?
  - Maybe they're not former lovers. As a spaceship pilot, Shep needs to stay away from the planet for long periods of time, just like any other soldier. They could be away from each other, but still be a couple.
I assumed this, partially because I missed the beginning.

It wasn't all a dream.

The story of the Crescendolls is so famous that they have a tremendous amount of merchandise. Note that the kid in the epilogue lives in a plain Earth home, but his clothes look... whatever-planet-the-Crescendolls-are-from-ese. After Interstella 5555 the two cultures began to intermingle... WPTCA Fese fashion became popular on Earth.

The only hiccup is the fact it is a Daft Punk record... my theory is that "One More Time" and "Too Long" are not actually Crescendoll songs, but Daft Punk songs, and are only used as part of the narrative. After all, if Daft Punk does exist in this universe... what did they put out if their (arguably) most famous album is from another band?

That brings up another thought, if Daft Punk exists in the movie's 'verse, it's hinted at, if not flat out stated that the Cresendoll's were not the first alien band that had been kidnapped, what if Daft Punk themselves are from another planet....that would actually explain a lot
Not really, in this movie's universe Daft Punk are probably earthlings and after the Gold Record Awards they heard about what happened to the Crescendolls and decided to tell their (Crescendolls) story to future generations, starting with their (the Crescendolls) song "One More Time". much of the music from "Aerodynamic" to face to face could be Daft Punk's songs inspired by the events told. While "Too Long" was another Crescendolls song they covered as the closing narration.

Octave was a member of the planet's stealth/intelligence force.

Related to the WMG above about Arpegius. Octave is the one that they use to get the memory discs back, and he's also seen doing the military salute at the unveiling of Shep's statue, when the vast majority of the crowd was not.

The movie takes place in the same universe as Cyborg 009

The character designs, kidnapping/brainwashing by badguys, and the fact that there are cyborgs just screams Cyborg 009. Hell, "Harder, Better, Faster, Stronger" sounds like they're being remodeled into cyborgs as well

Shep used to be or still was the lead guitar/singer of the Crescendolls

His ship is shaped like a guitar. The shape and color is also different from Apregerius. The reason why Earl didn't get him was because either he left the band before Earl caught wind of them; or he took a vacation. While buffing his ship he sings Digital Love in a style similar but not the same as Octave when he sang One More Time. To top it off he happens to know Stella and vise versa. If he was just a fan they both wouldn't be dreaming about each other.

The kid's father is Thomas Bangalter.

Think about it, the face of the father is not to be seen yet the mother is. He has all this Daft Punk and Crescendoll Memrobillia in his room, some of which looks more expensive than a single poster. According to articles, he has fathered a son. To top it off in the brief moment the mother is seen she looks exactly like Elodie Bouchez.

There will be a sequel some time in the near future

Arpegius and Stella Are Brother and Sister

They share the same hair color both in their alien form and in their humanized guise and in "Something About Us", he seems extra-comforting to her compared to the others.