The Dragonriders of Pern project. Bandied as a film then a series, and the last STRONG contender was under the old Atlantis Entertainment company that I think ... no longer exists? Like every couple of years it comes up again under some media company and then vanishes.

For a time Elfquest was bandied as a possible series, then dropped to a film, and finally I think it is dead in the water because they decided to do a radio play, basically.

I would have said Mr. and Mrs. Smith... but it is apparently actually happening!