This is a "Wild Mass Guess" entry, where we pull out all the sanity stops on theorizing. The regular entry on this topic is elsewhere. Please see this programme note.
Why did Taiji leave???
In one perspective, the one mentioned in Taiji's own retelling of the story, he and Yoshiki were fighting over the band's direction - he wanted more Hard Rock and Heavy Metal with less ballads, and these musical differences were why Yoshiki eventually kicked him out of the band.
In Yoshiki's retelling of the events, another perspective, hide and Taiji fought so much that hide asked Yoshiki to please tell Taiji to leave, and Taiji chose to quit the band rather than work out the issues.
Hide's hologram is actually a ghost.
Yoshiki summoned hide back from the dead for the reunion concerts. The hologram story was just a cover. Afterward, however, he stuck around, got bored and started causing trouble for Yoshiki, which was the real reason why the reunion had so much trouble. Eventually, however, Yoshiki was able to get hide to return to the afterlife and got Sugizo to replace him.
Some rumors, based on hide's public statements (specifically, that hide wanted to have sex as a woman and wished he could be reborn a woman, and that hide did not like anyone to see his body and would even swim clothed), have stated that hide may not have been satisfied with a male identity and/or preferred female or bigender or androgyne identity. Of course, these statements could also be attributed to hide's disgust with his body in general, and is therefore a pointless (and transphobes might say insulting) field of inquiry, but alternately... if he was really a she, wouldn't it be more respectful to refer to him as her or as a gender-neutral pronoun?
Which of the band members were substance users, if any?
hide was stated to have been a methamphetamine addict, according to rumors that the medical examiner who did his autopsy allegedly wrote a book with allegations of hide's blood tests and postmortem examination showing evidence of methamphetamine.
Arguing for this viewpoint is that the rush-crash pattern of methamphetamine addiction and its end-stage psychosis could have explained hide's mental disorders and angry combativeness, he was unnaturally thin despite the family genetics tending toward overweight, he could sing or talk VERY fast sometimes (the songs "Dice" and "Doubt" being huge examples of this, especially in live performance) and that he alluded to "speed freaks" in at least one song and one of the remixes of his songs is called "Tweaker."
Arguing against it is that if hide did indeed have Axis 1 Bipolar disorder comorbid to bulimia, this could have explained his mental and emotional conditions and unnatural weight loss just as easily as methamphetamine abuse could have, that despite his thinness, his appearance in general didn't seem to be one of an end-stage meth addict, that he could have simply been exploring the concepts of meth use in his songs/been intrigued by it but not a user himself, and/or used the drug without being an addict.
There are widespread rumors that Taiji had struggled off and on with either cocaine or meth addiction.
More Yoshiki drug rumors exist but they're probably best ignored, as they could be explained by substances he is legally prescribed and/or his health conditions equally as well as they could be explained by recreational drug use.
hide's death and theories on it
Driven to Suicide. The official story of hide's death was that he committed suicide. That said, this is highly disputed - most hide fans do not believe he intentionally committed suicide, Yoshiki and Taiji both insisted his death was not an intentional suicide, but the official verdict is that hide got drunk and decided to intentionally commit suicide for reasons unknown to anyone.
One variant on this theory is that hide's reason was to hurt and punish Yoshiki by forcing him to suffer through the pain of his lover's death, out of jealousy or anger over his relationship with Toshi. That said, this variant is pretty much only existent among one Yaoi Fangirl and her friends and associates - and spreading it is arguably trolling due to its inflammatory nature.
Medical treatment: Another non-intentional reason offered (usually by fans who don't want to consider the reason above), and also the theory that Taiji and Yoshiki both publicly put agreement behind: that hide suffered from neck pain as a result of the way he played his guitar, and had been instructed to use traction as a way to relieve the pain - and that he passed out drunk while in such a dangerous position.
Autoerotic asphyxiation: This theory posits that for some reason or another, hide decided to engage in an act of autoerotic asphyxiation once he was alone - and it went horribly wrong when he passed out either from alcohol poisoning or from the choking itself, accidentally dying as a result.
Never Suicide: There are a variety of theories that none of the above are correct, and that hide's death was neither suicide nor accident but rather murder.
One theory (often proposed by Trolls or Yoshiki's hatedom) is that Yoshiki was involved in hide's death. Like the variant of Driven to Suicide mentioned above, this one is so counter to the few objective facts that exist as well as, in light of how Yoshiki actually feels, a form of Kick Them While They Are Down, that it is not so much of a serious theory as a way for a troll to light up Internet Backdraft. Yoshiki was in Los Angeles at the time and had proof he was, he had no connection to the people around hide at the end of hide's life, and his own emotional reaction to the loss - all of this proves his innocence.
Another theory states that hide somehow ran afoul of the Yakuza in some way, and this led to his death. While slightly more plausible (especially owing to hide's Hair-Trigger Temper and that he may well have gotten into a fight with a yakuza at some point or other, and that if he was using methamphetamine he was likely dealing with them for his fix), it seems somewhat implausible for a yakuza hit in that most yakuza attacks are high-profile and meant to send a message (as opposed to being covered up as suicide). This theory is, though, plausible.
One more murder theory centers around the last two people seen with hide. The last people seen with hide before his death were a girlfriend and hide's brother Hiroshi, who was also his manager. Out of the murder theories, this one is one of the more somewhat plausible in retrospect. Hiroshi actually had a strong motive (financial, documented in almost pathetic detail over the last 13 years with his turning hide into a Cash Cow Franchise) and opportunity (being in physical proximity to hide without a strong alibi). Hiroshi and hide had been fighting a lot, and Hiroshi's actions in the aftermath of hide's death (demanding the investigation be closed for "privacy" and on a quick cremation of his body - then becoming an almost caricature of greed as he showed little actual grief for his own brother and far more interest in what money he could make from the fandom) all seemed suspicious...
Even if this murder theory is implausible, negligence to the degree of manslaughter can be proved: it is a matter of factual record, not theory, that both allowed hide to drink himself into a level approaching alcohol poisoning, then left him alone "to sleep it off" instead of getting medical help or staying with him to ensure his safety.
You Can't Fight Fate: This is another take on hide's death that ignores both murder and suicide theories, to state that his death was simply an inevitable tragedy resulting from both good and bad breaks in life and bad decisions that hide made, all of which led him to the point of death, and that however he may have died is irrelevant. In it, his body issues from childhood obesity developing into an eating disorder (or his unresolved gender issues), his being Axis 1 bipolar and not under proper care for it, his becoming The Alcoholic, his possibly developing a comborbid methamphetamine addiction, his success as a rockstar and the life that resulted from that in both good and bad aspects... all of it simply combined, as in how a driver's car, mistakes, road conditions, and wrong place at wrong time all caused a fatal car crash, and the Lunatic Loophole simply couldn't save him once more.
Taiji's death and theories on it
Official story: As stated in the media reports, and by Kitami Terumi and Shinso, the official story was that after his arrest for a fight with Ms. Terumi on a flight, he was jailed, and somehow hung himself with a bedsheet in jail, rendering himself braindead.
Police Are Useless, extreme negligence: This theory doesn't necessarily claim that foul play was involved in Taiji's death (one can entirely believe the official story of hanging by bedsheet while also believing this, and one could also believe in the full murder theory and that this was part of what set him up to be killed), but simply that US federal law enforcement in Saipan was highly negligent to not recognize someone in a vulnerable state and send him outside of Saipan to neuropsychiatric evaluation and treatment until trial, or immediately deport him back to Japan, or release him on his own recognisance to a friend awaiting him in Saipan, or even make sure his cell was properly suicide-proofed.
Never Suicide, murder (possibly with Police Brutality as the direct cause): This started out as a Conspiracy Theory based on rumors, but as the sources of the rumors have checked out, it has become a more plausible theory (to some, than even the official story). It relies on the fact that Kitami Terumi and Shinso had been embezzling money from Taiji, at the same time convincing him he had no choice but to stay with them. He had also sent one last e-mail to his apprentice Shu bemoaning his desperation with the situation. Once he was arrested after the fight on the plane, all that happened under "extreme negligence" occurred (this is not disputed), and somehow he ended up dead in his cell. Sometime during this, Kitami, using his phone, also tried to extort money from his fiancee. When his fiancee and mother saw his body, they saw no damage to his neck at all - despite the cause of death allegedly being hanging. Instead, he had tape marks on his mouth and a red slash on his chest, not wounds at all concurrent with hanging. They pushed for an autopsy, but Shinsho demanded and got possession of Taiji's body, demanding it be cremated with no autopsy done. This theory posits that whatever happened to Taiji could not have possibly been suicide by hanging in any way - and that whatever actually did happen to him was murder and the fault of Kitami, Shinsho, the detaining authorities, or all of the above.