News about the "Inhumans" movie cast has been scarce recently, as Marvel gives way to sooner releases and hypes them up altogether.
However, the season finale for another Marvel show, "Agents of S.H.I.E.L.D," have been viewed by fans to have given away a chunk of spoilers for the upcoming film, that is not due to come out until after four years.
It can be recalled that the season finale showed Simmons' character being sucked in a Kree crystal. It showed Agent Coulson and Agent Skye talking amongst themselves secretly. The two were discussing the possibility of starting a new team that would be built to deal with secret missions for the government. All with the intent of being anonymous the whole time.
According to Christian Today, although left unconfirmed by the producers themselves, fans still quickly assumed that the team which Coulson and Skye were talking about is actually the Secret Warriors, which is composed of Inhumans and other beings with super powers. Together, the Secret Warriors help S.H.I.E.L.D. combat evil forces that lurk the earth and threaten its peace. Much like The Avengers, but is different in terms of stressing out the importance of keeping their identities hidden.
Because of this, fans are speculating that the "Inhumans" movie cast would definitely include Skye, who is reported to be appointed as Secret Warriors' team leader, with Agent Coulson being the main organizer. Moreso, Sparkplug and Deathlok might also be included in the cast, should Coulson decide to bring them back.
Given the scene where Simmons was absorbed by the Kree crystal, there is also a possibility that she might be a part of the "Inhumans" movie cast, according to the site cited above. This is given that she would be able to free herself from the crystal and reveal that she acquired super powers through the incident.