While Marvel basically controls the game of cinematic universe's, it hasn't quite lived up to its promise on the TV side of things. Netflix excluded, Marvel's hasn't done much to build up a connected TV universe, badly losing that battle to DC and the CW. Marvel might finally be getting ready to catch up with the upcoming release of Inhumans, the ambitious project that will introduce the Inhuman Royal Family proper. The Inhumans, however, is a concept that already has a huge presence on Agents of S.H.I.E.L.D., so a crossover seems like a no-brainer, right? Well, Marvel may be looking to dip its toe rather than splash right in.
Inhumans is currently filming in Hawaii right now, but Agents of S.H.I.E.L.D. has already done a lot of the legwork introducing viewers to the concept of the secret community of super-people. The series has featured quite a few Inhuman characters, most notable main character Daisy Johnson (Chloe Bennet), so a crossover wouldn't be the most difficult thing in the world. Series co-creator Jed Whedon believes that they are going to hold back on a straight-up for a little while (via Digital Spy).
Jed Whedon later elaborates that Inhumans needs to build its own world, much like Agents of S.H.I.E.L.D. did, before jumping into any sort of crossover. In that regard, it makes sense to wait so that audiences can get a feel for Inhumans and build a deeper attachment to it so that a crossover would be more impactful. Whedon could just be keeping any plans on the down-low while they work out logistics, but his reasoning is certainly understandable.
And this isn't to say that there will never be a crossover. The CW Arrow-verse crossovers are ratings mega-hits, so it's only logical that Marvel and ABC would want to capitalize on that. Jed Whedon later hints the merits of a S.H.I.E.L.D./Inhumans crossover and how the shows can still emotionally connect, if not physically.
Fans should expect some kind of connection between the two shows. Whether that means a quick line from Coulson referencing a trip to Hawaii or Inhumans dealing with their new public existence (S.H.I.E.L.D. made the Inhumans public knowledge), no one if forgetting that these two shows exist in the same world. It'd be really crazy for Marvel not to take advantage of this situation, so a crossover is an inevitability -- just not a fast one.
Inhumans will premiere on IMAX on September 1, 2017, and run for two weeks before airing on September 26 with an eight-episode run.