@tenyari
The whole reason there's a never released Fantast Four movie from a few decades back was the movie studio wanted to prevent the rights from reverting back to Marvel (a moot point now as the studio and Marvel are now both under Disney).
Actually, that is an issue of owning film rights.
When a company buys film rights, one of the stipulations is that they actually produce a film or TV series in a set period of time, or they revert back to the owner.
In 1986, West German company Constantin Film bought the rights to make a FF movie, and they were given 6 years to do so. In 1992 the rights were about to expire, so he hired Roger Corman to film a cheap movie known as an "Ashcan Print", simply to maintain the rights.
And that is not unique. As part of the contract for "Blazing Saddles", Warner Brothers maintained the sequel rights so long as they used them. And years later when Mel Brooks announced he wanted to do one, WB informed him they still had the rights, as they had indeed made a TV series.
They filmed an entire season of "Black Bart", with Lou Gossett Jr and Steve Landesberg as the leads, but only aired the pilot a single time in late night. However, that fulfilled the requirements even if they never intended to ever air it.
Disney only reacquired the fights to FF because they bought Fox Pictures. If not for that, they would still be the rights holder. Just as Sony still holds Spider-Man.
But I would love to see an FF set in the Multiverse. Where they could have the newest Johnny Storm meet Captain America and Killmonger.