Forever Love. It's made it everywhere from an anime to a band member's funeral to a political campaign ....
For the fandom - "Alive", which is a guitar-driven ballad as opposed to the usual piano-based ballads the band has. The fanbase has demanded countless times in recent years for Yoshiki to revive the song live.
Cash Cow Franchise: At least with touring and the sales of rereleases and the like, enough to keep the band together.
Creator Breakdown: Everyone except Heath and Sugizo has seemed to have had at least one:
Yoshiki has had six, as mentioned on his page and elsewhere. Both physical and mental.
Toshi had one though it spanned roughly 11-12 years in happening: becoming triggered and depressed from too much work and too much focus on Yoshiki's dark lyrics, getting involved with Kaori Moritani and marrying her, then joining her Scam Religion and becoming brainwashed and finally Brainwashed and Crazy over his years of involvement with it, before finally walking away from it.
hide became bulimic, The Alcoholic to an extreme degree, and a likely methamphetamine user, along with potentially suffering from Bipolar 1, and accidentally committed suicide due to the confluence of alcohol, drugs, and the absolute lack of care anyone in Japan at that final night of his life had for him.
Taiji began to suffer from a dissociative disorder and went Off The Wagon with alcohol and either cocaine or amphetamine-class stimulants in the mid-90s when he left the band, the combination of which left him homeless and destitute (and as a result suffering an attack that damaged his face and nearly killed him at the time). He had a stroke that uncovered temporal lobe epilepsy, also likely as a result of stimulant abuse. He struggled with this and the effects of it, and eventually was likely murdered in Saipan and had his death implausibly covered up as a suicide.
Pata, despite being the Only Sane Man for the 1987-1998 years, became The Alcoholic himself more and more over the years, especially after hide's death.
Dawson Casting: Most likely used in Dahlia PV as the "young girl" there could have been 18 or 19 and young looking. Averted in Celebration where the protagonist was actually a nine year old playing a thirteen year old fan of the band, and in the open of On The Verge Of Destruction (the girl reading the book).
Old Shame: Vanishing Vision, for the cover that seemed to depict rape as erotic and sexy. Engaging in the Bar Brawl, property damage, assaulting and threatening people, and similar behavior.
For Sugizo, smoking tobacco.
For Toshi his years with Home of Heart, and his music created for them.
What Could Have Been: In 1991-early 1992, X Japan was set to break into the American Heavy Metal scene in Los Angeles after a planned show at Madison Square Garden in New York City. The US release of Blue Blood and gigs were planned - then a confluence of events ensured that the band would not return to the US to perform for over 20 years. It began with a Frivolous Lawsuit by the LA punk rock band X over their name - which forced them to change the name to X Japan and not perform in the US. Next, Toshi developed vocal problems related to vocal polyps and no US label was willing to take a chance on a vocalist with a Japanese accent. Finally, the issues between Yoshiki and Taiji hit their peak, and Taiji was kicked from the band - leaving it without a bassist.
One last attempt to debut Art of Life at Madison Square Garden in 1993 once Toshi's vocal issues were better, Columbia Records agreed to front, and Heath was the new bassist failed because by that time, Heavy Metal had experienced a major backlash and popularity reduction with the collapse of Hair Metal - which, being Visual Kei, they unfortunately looked too much like even toned down, and sounded too much like for the average listener of the day due to not being Grunge - no one "serious" was willing at the time to take chances on metal.
An earlier reunion was supposedly planned (to begin in 2000) without Toshi but with hide taking over as lead singer. hide's Author Existence Failure prevented that from happening.
Miyavi as lead guitarist instead of Sugizo. While there were other candidates for the lead guitar position (including Wes Borland and Richard Fortus, who played as hired guns in at least one gig, and Kaoru of Dir En Grey), the final choice came down to Miyavi and Sugizo. Sugizo was chosen of the two and has performed admirably as the lead guitarist, but Miyavi would have brought an entirely different and more youthful creativity and energy to the band more similar to hide's improvisational skill so it really is a tossup as to which one would have been best.
Taiji Sawada's return to the band. Had he been brought on as a full member for all tour gigs in 2010 and 2011 (including the US tour), he would likely not have ended up in Saipan (and had he still gone, he would have had far more to work with to save himself). The band itself would have also sounded so much better as a double bass band featuring him and Heath both - the Yokohama performance showed just how much they complemented each other. Instead, with whatever happened, he ended up dead at the hands of either himself or someone else in Saipan, and the band is much poorer for him not being there.
The biggest one of both: a combination of the two preceding: Miyavi having been chosen as lead guitarist and Taiji as bassist or second bassist. The combination would have put a ton of new life into the band, giving it three members capable of full attention and creative input including the most innovative person in the modern Visual Kei scene and the person widely known to be second only to Yoshiki in the original development of X itself. Taiji would have likely never signed on with KT and never gone to Saipan and not died as a result, and it's arguable that X Japan would have been far different and yet far more active as a band and re its individual members. It's only speculation, but it could be arguable that Miyavi and Taiji and Yoshiki could have had a similar synergy as hide and Taiji and Yoshiki did in the first iteration, and if so, this could have had some very interesting results - and that it can't happen now because of Taiji's death makes it one of the saddest possibilities.