In an interview with the BBC, Karen Gillan talked about her upcoming feature film directorial debut TUPPERWARE PARTY, as well as a few of her other upcoming projects. Among those upcoming projects, she let slip that she'll be reprising her role as Nebula in AVENGERS: INFINITY WAR for "a little appearance".
With the cast of the next AVENGERS film apparently ballooning to almost 70 characters, it's not too surprising that she'd return (hell, I imagine they might even bring back Tim Blake Nelson's The Leader at this point. Remember him?)
What is interesting, is that Nebula actually has a fairly substantial role in the INFINITY GAUNTLET arc in the comics (of which INFINITY WAR is ostensibly based on), which makes sense too, as she's the adopted daughter of the big bad Thanos. Hell, it's already been confirmed that Zoe Salanda's Gamora would return as well. However, Gillan saying that Nebula's role is nothing more than "a little appearance" seems to suggest some major changes to the storyline. Though that's been the case for all of the MCU adaptations.
Either way, AVENGER: INFINITY WAR starring everyone (I think even I might be in the movie at this point) will make all the money May 4th, 2018, and you can next see Karen Gillan's Nebula in GUARDIANS OF THE GALAXY, VOL. 2 almost exactly a year earlier May 5th, 2017.