It was mentioned in the first page - Spielberg's Night Skies. It may have never been made but the ideas were used in both Poltergeist and E.T. the Extra-Terrestrial.
IIRC, Rick Baker originally designed a working E.T. and other E.T.s that were scarier. Baker posted photos of his efforts that cost him a couple of thousands dollars, on his twitter earlier this year:
https://***********/TheRickBaker/status/469494395813638146
Would have made for an interesting E.T. film instead of the one we got in 1982.
There is this one unmade film by David Lean - Nostromo. An adaptation of Joseph Conrad's 1904 novel of corruption and greed in a fictional South American republic. It would have been made if not for Lean's ill health, creative disputes, and a spiralling budget. Lean died about a month before shooting was due to begin.#
Others of note, David Lynchs head spinning Ronnie Rocket in the wake of Eraserhead.
David Cronenberg's - Frankenstein.
Andrei Tarkovskys - The Idiot based on Dostoevskys classic
And of course the 4 Superman films that never took off:
Tim Burton/Kevin Smith - Superman Lives (after rejecting the script for Superman Reborn)
McG/ J.J. Abrams - Flyby (They must have hated Harry Knowles LOL). In this version, IIRC Ashton Kutcher - Superman, Scarlett Johansson - Lois Lane. Shia LaBeouf - Jimmy Olsen and Johnny Depp - Lex Luthor.
Wolfgang Peterson - Batman vs. Superman
George Miller - Justice League.