Marvel's Inhumans finished Season One with very poor ratings and reviews, but that didn't stop the show from attempting to build a connection to the larger Marvel Cinematic Universe by teasing a larger over-arching villain.
In the finale, we learned a secret chapter in the backstory of Black Bolt: it was Maximus who originally tricked Black Bolt into the emotionally traumatic event that led to him accidentally killing their parents. Maximus figured that deep trauma would push Black Bolt away from the throne, and it would have, if not for one key event that changed Black Bolt's mind. Specifically, when the head of Attilan's Genetic Council revealed to Black Bolt that a great enemy threatened the existence of the Inhumans, and it was the king's job to protect the people from that threat.
It's only teased in The Inhumans finale episode, but as Black Bolt ominously warns the Royal Family towards the end of the finale, moving the Inhuman community from Attlian (on the moon) to Hawaii (on Earth) was equivalent of hopping out of the frying pan, into the fire. With that in mind, let's break down what kind of larger enemy in the Marvel Cinematic Universe could be considered a major threat to the Inhumans:
The Mad Titan (Josh Brolin) has been the overarching villain for the Marvel Cinematic Universe ever since the end of The Avengers, and will finally make his full debut in Avengers: Infinity War next year. Since Infinity War will be pulling together ever corner of the Marvel Cinematic Universe for a war of Cosmic proportions, it would make sense that Black Bolt and the Inhuman Genetic Council would have knowledge that Thanos, the Infinity Gauntlet, and Infinity Stones all represent to the universe.
How Likely Is It? Marvel TV series aren't guaranteed to have any major crossover with Infinity War, which means it's not very likely that Inhumans, the worst-rated entry in the entire MCU, would get a cameo in the film. It's not impossible, just very unlikely that Black Bolt and Thanos ever come face-to-face in the MCU.
The Inhumans are a race that resulted from the Kree experimenting to unlock super powers in ancient humans; therefore, the Inhumans also share a common enemy with their Kree creators: Skrulls, a shape-shifting race of aliens that have been at war with the Kree for centuries. This past year, we learned the Skrulls will be making their presence officially known in the upcoming Captain Marvel movie starring Brie Larson, which will detail an attempted Skrull invasion back in the 1990s. The Skrulls work in clandestine ways, infiltrating societies and destabilizing them by replacing key figures with Skrull impostors. In 2000s Marvel Comics, the Skrulls activated a decades-old plan to invade earth, revealing that key Marvel Universe heroes and figures had been Skrulls all along.
How Likely Is It? A lot of MCU fans have speculated that after Thanos and Infinity War, the Skulls and their "Secret Invasion" storyline could be the basis for the next big overarching story in the MCU. If that's indeed the case, then Inhumans could easily be planting the early seeds for that event. Black Bolt and the Genetic Council would likely be some of the first figures in the universe to know that the Skrulls were plotting revenge against Earth, after the events of Captain Marvel.
Agents of S.H.I.E.L.D. Season Five trailers have revealed that Agent Coulson and his SHIELD faction are headed into space to stop an intergalactic threat, and some early speculation is that the threat could be the Brood. The Brood are a race of evil insectoid aliens, which infect they're enemies with eggs that transform them into new members of the Brood. If the hosts have superpowers, the new Brood soldier inherits them; in some cases, Brood retain their host's memories and personalities, or are even able to switch between their host form and Brood form. In short: they're a deadly cosmic infection, created in the vein of the Alien franchise's Xenomorphs.
How Likely Is It? In terms of options, a cosmic-based Agents of S.H.I.E.L.D. storyine has the best bet of crossing over with the Inhumans TV series. Of course, that all depends on what the nature of AoS Season Five's alien threat truly is. We've seen Kree soldiers featured in trailers, already making the case for an Inhumans connection -- and given how poorly Inhumans was received, this may be the only crossover bet Marvel is willing to put on the table.
Inhumans Season One is now available on streaming and on-demand services. Agents of S.H.I.E.L.D. Season Five premieres on December 1st.