Avengers 4: Every Character Confirmed So Far
2. Mantis
Confirmed By: Pom Klementieff
Though she might not be a full-on Guardian Of The Galaxy yet (simply because she wasn't really co-opted into the team during the sequel), it seems likely that Mantis will be back for Avengers 4 even though she was killed.
Pom Klementieff became the latest Guardians actor to confirm that she's back for Avengers 4 when she took to Facebook to tease a make-up trailer shot from the Avengers set. And since we know they were filming Avengers 4 at the time - and not Infinity War, despite the hashtag - that's what she's preparing for here...