"Agents of S.H.I.E.L.D." season 2 ended with Simmons getting sucked into a Kree crystal and Agents Coulson and Skye talking about building a "secret" team that may or may not be comprised of Inhumans. For fans of the comic books that the series is based on, this is in direct reference to the Secret Warriors. 

The Secret Warriors are a band of Inhumans and other super-powered beings working directly for S.H.I.E.L.D. Skye, who is known as Daisy Johnson a.k.a. Quake in the comics, leads the group as they tackle secret missions for the government. The Secret Warriors tend to focus on covert operations and always remain anonymous, which is the complete opposite of the Avengers. 

According to a report from Den of Geek, it is speculated that Coulson will be the main organizer who will spearhead the formation of the Secret Warriors, while Skye will be appointed as the team's leader. Luke Mitchell's Sparkplug may be brought in as a member and so could Deathlok if Coulson brings back Michael Peterson. 

The season 2 finale ended with Simmons getting absorbed by a Kree crystal but if she comes out, survives, and reveals she now has super powers, then she might be another addition to the Secret Warriors. 

However, all of these are mere speculations, and it is important to note that executive producer Jeffrey Bell previously told IGN that they will introduce a new group in season 3 but they may or may not be called the Secret Warriors. This does hint, however, that the Secret Warriors may be included but will don a new name. 

Den of Geek speculates that the introduction of Inhuman agents and the formation of the Secret Warriors will directly lead to the events in "Inhumans," a Phase Three film in the Marvel Cinematic Universe. "Inhumans" is scheduled for release on July 12, 2019.