Only "Ghosts" and "Is It Scary" have the same theme, which is occult stuff.
"Blood on the Dance Floor" is about AIDS a female killer.
"Morphine" is about drugs.
"Superfly Sister" is about being a prude. Not Latoya like some people think. The only reason it's on the album is because "In the Back" was taking too long to finish.
You could lump "Morphine" with "Ghosts" and "Is It Scary" as it's about MJ dealing with his personal struggles, but so is "On the Line".
All of the five original songs were unreleased before being included on the album. "On the Line" was recorded in 1996 and released as a promo single in 1997, the same time the album came out, so it'd fit.