The recent news that Marvel's Inhumans is getting pulled from its July 12th, 2019 release date came as a shock to most fans considering there was no real reason given for the decision. Marvel has been going out of its way to build the Inhumans lore up in Agents Of S.H.E.I.L.D. just to prepare for the film in 2019, so it makes no sense as to why they would just let go of their plans.
What could have made them change their plans all of a sudden with no indication as to why? What could they feel was so important that they would have to actually open up the slot that Inhumans was occupying? Could Marvel and FOX be working on an imminent deal that would bring the rights to the Fantastic Four back to Marvel?
FOX has shown their incompetence when handling almost any superhero franchise. They have managed to keep X-Men afloat, but they were not so lucky with 2015's Fantastic Four. It has been rumored for some time now that FOX and Marvel would make a deal to share the film rights to the Fantastic Four similar to the one Marvel and Sony completed for Spider-Man's film rights.
In fact the last time Marvel pushed back its film slots was when the deal for Spider-Man to join the Marvel Cinematic Universe was made. They ended up pushing back Thor: Ragnarok, Black Panther, Captain Marvel, and Inhumans in order to make room for such a popular character. Getting the film rights back for a popular brand such as Fantastic Four would certainly entice Marvel to hold off on their original plans for a smaller brand like Inhumans.
It is just a theory at this point, but it would make sense. You know Marvel would not just pull a film and leave that date completely open. It's possible that they got an idea for one of their other properties and are simply moving that film forward. Marvel announced they have three unnamed films planned with release dates of May 1, 2020, July 10, 2020, and Nov. 6, 2020; so they have slots open if they had wanted to make room for other projects.
With those three other dates available in 2020 it makes little sense to not simply schedule any current property Marvel owns in one of those three slots. It would make sense however that they had an unexpected opportunity like getting the rights to Fantastic Four back, and are making room for a juggernaut addition to their cinematic universe.
Whether the Inhumans cancellation and Marvel's interest in obtaining Fantastic Four are connected or not, hopefully FOX stops holding the Fantastic Four hostage and learns they can make more money by letting them join the MCU. FOX still has the X-Men universe to make money off of so there is little point in them continuing to keep the Fantastic Four now that it will take them years to bounce back from their failures with the last Fantastic Four film.
Maybe we will also still get an Inhumans movie; Marvel could move Inhumans back to one of the three dates in 2020. Considering how far they have gone to build the Inhumans up on TV, it is unlikely that they will abandon the property altogether.
This also could just simply be a result of the dispute between Kevin Feige and Isaac Perlmutter. Perlmutter is currently head of the TV side of the Marvel Universe and since the Inhumans are largely part of the TV aspect of the things, it is possible that this is what caused the move. I think it is too much of coincidence with FOX and Marvel agreeing to allow FOX feature the X-Men on TV in the upcoming show Legion.