I'm sure this topic got raised 10 years ago when The Solo Collection came out (has it been that long?!), but I can't be bothered trying to find it. So, here it is fresh.
The following is the best list I could come up with as to what did not make it into the boxed set. I excluded songs where he just came in to sing back up, because those really go beyond the purpose of the set. Collaborations get a little sticky, so when in doubt, I included them. Lastly, not every version I list was really needed on the set, but I'll leave it up to you to decided what should have been included. Some tracks definately warranted inclusion!
Here we go...
(Post Edit) “State Of Shock (Demo with Michael Jackson)”
(Post Edit) “There Must Be More To Life Than This (Demo with Michael Jackson)”
(Post Edit) “Victory (Demo with Michael Jackson)”
“Love Kills (Queen Demo)”
“Love Kills (Wolf Euro Mix)”
(Post Edit) "Man-Made Paradise (Queen Demo)"
(Post Edit) "There Must Be More To Life Than This (Queen Demo)"
“Made In Heaven (7” Remix)”
“Living On My Own (US Extended Version)”
“Mr. Bad Guy (Bad Circulation Version)” *
“Mr. Bad Guy (Malouf Album Mix)
“I Was Born To Love You (Disconet Remix)”
“She Blows Hot And Cold (Original Version)”
“Exercises In Free Love (Montserrat Caballe Vocals)”
“Africa By Night (Demo)”
“Barcelona (TV Edit)”
(Post Edit) “Barcelona (Gold Version)” = N/A its likely the Single Version
“The Golden Boy (7” Edit)”
“The Golden Boy (Radio Edit)”
“The Golden Boy (Tim Rice Edited Version)”
“The Fallen Priest (7” Edit)”
“Guide Me Home (Dutch Single Version)”
“Living On My Own (Techno Mix)”
“Living On My Own (LA Mix)”
“Living On My Own (Dub Mix)”
“Love Kills (Fugitive Brothers Radio Mix)” ***
“Love Kills (Symphony Mix)” ***
“Love Kills (Jazzy Mix)” ***
“Time (2000 Remix)”
“In My Defence (2000 Remix)”
“The Man From Manhattan (Revisited)” (Eddie Howell, Freddie Mercury & Brian May)
“The Man From Manhattan (Back Again)” (Eddie Howell, Freddie Mercury & Brian May)
"You Really Did Me In" (Ian Hunter with Freddie Mercury, Roger Taylor & Brian May)
“Emotions In Motions” (Billy Squier with Freddie Mercury & Roger Taylor)
“Love Is The Hero (Album Version)” (Billy Squier & Freddie Mercury)
“Love Is The Hero (Reach For The Sky Extended Version)” (Billy Squier & Freddie Mercury)
“Time (Album Version)”
“Time (Reprise) (Album Version)”
“Foolin’ Around (Teachers Version)”
“Heaven For Everyone (Roger Taylor vocals)” (The Cross with Freddie Mercury on backing vocals)
* This demo would eventually be released in 2006 on The Very Best Of Freddie Mercury Solo double CD set.
** This claims to be on the Instrumentals disc, but it’s actually the “Exercises In Free Love (Instrumental).”
*** Mixed by The Fugitive Brothers (who also did the “Love Kills (Rock Mix)”, but never issued).