The Inhumans movie is the last one scheduled in Phase 3 of the Marvel Cinematic Universe, coming in 2019. It's a pretty obscure concept – just like Guardians of the Galaxy was – and Marvel Studios will be hoping the race of genetically altered humans can become their "X-Men", but that probably won't be the case.
Inhumans will be decent enough, but without the humour of the Guardians of the Galaxy and the established popularity of the X-Men, it certainly won't blow people away.
Moreover, after the second part of the Infinity War saga, it might feel like something of an anti-climax and probably isn't the best way to round off Phase 3.
The likes of Black Bolt, Medusa, Gorgon and Karnak are interesting and varied enough (especially in terms of their powers), but they just don't have the selling power or impact that most other offerings in the third phase of the franchise will have.