I find this to be an interesting thread. And honestly, up through Dangerous I don't know if there's really any songs I would add. The way the albums are now, they have their certain personalities. And tampering with that would change everything. But anyways I guess I'll go album by album and explain what I might have done.
I suppose I would add "Sunset Driver" to Off The Wall just because I think it's one of Michael's best up-tempo early songs. Although obviously it's got nothing on "Don't Stop 'Til You Get Enough" or anything.
For Thriller I really don't think there's a song I could remove. However, I do think "Behind The Mask" should have been released somehow. Maybe as a stand alone single. It would have been an absolutely HUGE song and there is no doubt about it. It just makes me vibe every time I listen to it. And those sheets of synth are just ear candy. And I could believe that removed the awesome synth from the Michael album!
I don't know if we can add Jacksons albums to the discussion, but I do have to say that there should have been a few more Michael songs on there. I suppose he was saving songs for his follow up to Thriller. However, I still think that as far as stuff that we know for sure was recorded, it couldn't have hurt for Michael to release "Buffalo Bill" on Victory. Supposedly it featured his brothers singing more and not only Michael but Bruce Swedien and Matt Forger have had great things to say about it. Shame it wasn't on there rather than something like "Body". Yuck. And I have to assume that "There Must Be More To Life Than This" and "Victory" were likely intended to be on the album. Wouldn't have minded either of those being on there.
As far as Bad, I think maybe I woulda lost "Man In The Mirror". I know, I know say what you will about me. But I don't care for it. I think something more understated would've worked in it's place, such as "Scared Of The Moon". And as far as a different up tempo song, I really would've liked "We Are Here To Change The World" to be on there in place of "Speed Demon" or "Just Good Friends". I think it would've easily fit in there in place of either, as it's a dancey tune with a positive message.
With Dangerous there are definitely a few changes I'd make. First off, I'd have put "Messin' Round" on there, as it's been shown this was worked on during this era. I think it woulda added more of a sense of fun to the album. Perhaps it could've taken the place of "Give In To Me" or even the title track. I feel like the album, as magnificent as it is, could've used a bit more "lightness". And I think a fun dance song like this woulda been great. Especially as I imagine it enhanced for the '90s by Teddy Riley's production. I also think "Serious Effect" would have helped the album, perhaps in the place of "She Drives Me Wild" or "Can't Let Her Get Away". I think it's super funky and had more of the edgy, "street" sound Michael was looking for. As far as ballads, obviously I'm unsure of what all they had to work with, but I would've probably liked to have "Someone Put Your Hand Out" and "For All Time" finished and released on the album instead of "Heal The World" and "Will You Be There". Once again, I'm sure I'm gonna get shit for this, but I think "Heal The World" and "Will You Be There" are two of his most overrated ballads and the two I picked are much more understated and fit the feel of Dangerous better in my opinion. I feel like "Heal The World" and "Will You Be There" really didn't fit with the spare, slightly minimal feel of a lot of the other material on the album.
As far as HIStory, it's hard to say since we don't have or know much as far as outtakes go. I will say that even though it was apparently never seriously considered, I would have definitely added "Much Too Soon", perhaps instead of "D.S.", which although accurate in my book, kinda wasn't a good look for Michael.
Lastly, Invincible to me in it's current state just seems like a grab bag of songs Michael had been working on, with no real coherence. So due to that, I'm not really as worried about flow and whatnot. But regardless it is ridiculous that "Chicago" wasn't on there. It's an amazing song in the original version, and while it probably wouldn't have been a hit, it should have (and probably will) go down as a classic moody MJ track. I do think rather than "Threatened" the sign-off track should've been "We've Had Enough", with it's powerful message and Michael's amazing performance. Once again, I'm not sure if this one would've been a hit, but it should've been a single, especially coming after 9/11. If it was indeed finished before the release of Invincible (which I'm unsure of) it shoulda been there. I also think if "One More Chance" should've been on there as well if it were finished in time. Once again, I'm unsure whether it was finished until the 2003 release of Number Ones or not. I know it might not have been exactly what Michael wanted, but I think an album featuring classic, slightly retro stuff like "You Rock My World", "One More Chance", "Break Of Dawn" and "Butterflies" on one side and then more modern stuff like "Unbreakable", "Xscape" and "Chicago", along with the classic anthem "We've Had Enough" (rather than the weak "Cry") would've been absolutely amazing, likely garnered better reviews, and shoot, maybe even spawned some more hits.