No plot details have been confirmed as of yet, but we do have an idea of where the story could go for subsequent seasons. The season 1 finale saw Cassie teaming up with Miranda (Gomez) to take out Felix (Colin Woodell) once and for all, and in the season's final moment, it appears as though Cassie's been recruited by the CIA, thanks to Shane being involved with the organization from the start. It's possible we'll see Cassie sticking with her flight attendant job, while also working for the CIA. It would be intriguing to see her spying on people and getting wrapped up in espionage all while trying to stay sober.
Meanwhile, Miranda ran off with a stash of cash and is nowhere to be seen. Miranda actually dies in the novel, but the show took a different direction with her character arc. With a second season now in the picture, it's possible she could meet a violent end if she runs afoul the wrong people.
There are plenty of directions The Flight Attendant could go for many more seasons, but one thing remains certain — whatever mishaps Cassie Bowden gets into next, we'll be watching.