I was reading this today and thought of this thread.
I Don't Want To Miss A Thing by Aerosmith was originally written for Céline Dion.
I can just imagine Céline singing this power ballad.
Don't You (Forget About Me) by Simple Minds was offered to The Fixx, Bryan Ferry, and, finally, Billy Idol, all of whom turned it down.
Hungry Heart by Bruce Springsteen was almost a punk rock anthem instead of the his first top ten hit. Joey Ramone had asked Springsteen to write the Ramones a song, and he did, but decided afterwards to keep it for himself on the advice of his manager.
How Will I know by Whitney Houston was written for Janet Jackson who turned it down.
Holiday by Madonna was offered to Mary Wilson.
Wow, how different this would've sounded!