Here are a few ideas I've had about some random things. They're probably all rubbish, but here they are anyway:
Meteors (19th August 2014)
I haven't seen anyone else say anything about meteors before, so I feel kind of stupid talking about meteors, but I think that there's a fair amount of evidence to back it up. In Heaven Piercing Giga Drill, it says "All the lights in the sky are stars", and in the 4th movement of Bōsōzoku Symphonic, it says "All the lights in the sky are falling to the ground", so it's basically saying that "Stars are falling to the ground" = Meteors, right? There's also some evidence in other lyrics:
Euphemia: "As I looked into the burning sky" - The sky would be filled with flaming meteors.
The Strays: "Do you remember when the stars were much higher" - The meteors would have been high up in space before, but now they are falling down to earth.
Fear of Cancer: "Well the skies collapsed as the gas diffused" - Meteors are made partly (or maybe mostly, I don't know xD) of gases like Carbon Dioxide and Methane.
Echoplane (19th August 2014)
This is just a really random, crappy idea I had, and there isn't any evidence to back it up. I think that Echoplane is an imaginary place inside Cassandra's mind made of memories. Cause memories are just an echo of what actually happened. And maybe she's the one in "a restless sleep through an IV high" in Vectors, and while she is asleep she is in Echoplane re-experiencing all her memories. I don't know, just a random idea I had xD
IV from Vectors mentioned in Bōsōzoku Symphonic (19th August 2014)
In Vectors, it says "A restless sleep through an IV high", suggesting that the person (I think it is Cassandra) is subdued through drugs coming through an IV (needle). In the 4th movement of Bōsōzoku Symphonic, it says "I'll rip out all the hooks from my skin so I can grow". The "hooks" could refer to the IV in Vectors, and Cassandra wants to get the IV out and wake up.
Second Echoplane Theory (Later in the day of 19th August 2014)
After listening to closer to some lyrics, I've come up with another theory about what Echoplane is. It's VERY similar to the first one, except this time I think it is basically the land which people go to when they dream (rather than being a world of memories). Cassandra and Sparkles* share an Echoplane, except that Cassandra is permanently stuck there because of the IV from Vectors, but Sparkles* only goes there when he sleeps. There are loads of references to dreams in ATLITS, but here are the main pieces of evidence that gave me this idea (they are all from Cassandra pt II):
"In dreams, I'm coming to find you" - Sparkles* can only try to find Cassandra when he is in Echoplane (dreaming).
"Cause we are the same" - They have the same dream world.
"We'll run around in secret worlds and say our secret words" - He's talking about Echoplane when he says "secret worlds".
Ghosts (20th August 2014)
There are of course a few very obvious references to ghosts (GhostOfSparkles*, Ghost Cassette), but I think that there may be more significance to the theme of ghosts than first meets the eye. There are two more references to ghosts that I've noticed (they might have already be found, but oh well):
If you look only at the black part of the eye in the Area 11 eye logo, it look almost exactly like a ghost.
Knightmare/Frame: "Ichiban, Zero (First, zero)" - Sparkles* said on his DeviantART that 1 is whole and 0 is nothing. If you are in-between being and not being, then you are a ghost.
Cassandra's name (6th October 2014)
I think I've been on the Digital Haunt wiki a bit too much today, and this theory is so ridiculous, and is probably just a coincidence. But, I'll share it anyway:
So, there's been an obvious theme around the number 3 in the Digital Haunt, eg.
Legendary Sannin: "We were THREE and now we are one" All the Lights in the Sky: "There's THREE parts to love"
So I just realized that Cassandra's name has 9 letters, which is 3 squared (3 lots of 3). If you split her name into those three parts, you get: CAS SAN DRA
The 'CAS' could possible relate to Ghost CASsette, and 'SAN' means 3 in Japanese (Legendary SANnin means Legendary 3 people)
I have no idea what the 'DRA' could mean though... DRAcula? xD
I have no idea what kind of relevance this has, but I'm delirious from being on here for too long, so don't judge me xD
Collection of references to lines (22nd October)
Vectors are lines.
Vectors: All the LINES between the dots
In the Blind: Hold the LINE
BlackLINE EP UnderLINE EP RedLINE (Bosozoku Symphonic Movement 3)
References to Theatre/Films (6th November)
Hypoxia- "The whole world is a stage"
The Strays- "Cut to the next scene"
In The Blind- "Thespianic Answers" (Thespianic means relating to acting/drama)
Cryogenic Statis/Dream World theory (22nd November)
After the release of Underline, this is my updated theory:
Like a lot of people, I think that Cassandra is being held in Cryogenic Stasis. This means that, in a way, she's permanently asleep, which links back to my second Echoplane theory. So, Cassandra is always in the Echoplane, but Sparkles* only goes there when he sleeps. I think that something has gone wrong in the stasis chamber, because in Override it says "Under the ice"., and in Cryogenic Stasis, ice forming means something's gone wrong. So Sparkles* is trying to communicate with Cassandra through the Echoplane, and telling her to override the system to stop the stasis chamber and get out. I also think that he might be telling her to explode, to break the stasis chamber and to get revenge on whoever put her in there (Inertia Corp?). Here's a bunch of lyrics which provide evidence for this theory:
Knightmare/Frame: "Inside a machine" - Cassandra is being held inside a stasis 'machine'
Override A: "Under the ice" Ice has formed in the chamber
"Dihydrogen prism of fate" Stasis chambers have some dihydrogen (water) in them.
"The life giver will take away" Usually, stasis chambers save lives,but, since something's gone wrong, now it will kill Cassandra if she stays there.
"You'll claw yourself out of the womb" Stasis chambers can be used for babies, so they are a kind of 'womb'. So Cassandra will escape the chamber?
"Are you willing to die?" Cassandra will die if she explodes.
"I won't let you drown" Sparkles* doesn't want Cassandra to drown in the chemicals in the stasis chamber.
Scissorborn: "I hear this calling me, to break this hibernation that's killing me" - Cassandra can hear Sparkles* telling her to get out of the chamber (hibernation) which will kill her if she stays in it.
B612 (16th December)
This may be something completely unrelated to the DH, but, in this instagram photo posted by Sparkles*, he used an app called B612 to edit it (it's written in the corner). After a quick Google search, I discovered that this is also the name of a private foundation which is based on planetary defense against asteroids/other threats in space. This not only ties in with the theme around lights and skies, but also with my theory about meteors.
Those are the only somewhat sensible ideas that I've had so far, I'll add more as I think of them, please tell me what you think of the current ones in the comments! :D