SPOILERS for the Agents of S.H.I.E.L.D. season 5 finale ahead.
According to Agents of S.H.I.E.L.D. co-showrunner Jed Whedon, Graviton may not be as dead as he seemed after the season 5 finale. Agents of S.H.I.E.L.D. first hinted that comic book villain Graviton would be appearing on the show all the way back in season 1, but didn't actually pick that plot thread back up until this year. Fortunately, the Graviton reveal proved to be worth the wait as, in an unexpected twist, General Glenn Talbot - a longtime recurring supporting player and frenemy of S.H.I.E.L.D. on the show - ended up being the one to inject himself with loads of Gravitonium and thus, transform into the super-villain.
Talbot/Graviton also turned out to be the true "Destroyer of Worlds" and the one responsible for cracking the earth like an egg in a dystopian future that, thankfully, the Agents of S.H.I.E.L.D. were ultimately able to prevent from happening. The season 5 finale in turn concluded with Daisy Johnson besting Graviton in a fight by "quaking" him into outer space and seemingly killing him in the process (since, as powerful as he had become, Talbot still needed air to survive). Of course, Marvel Cinematic Universe fans should know better by now than to assume that anyone is for sure permanently dead and gone.
Whedon said as much when EW asked him if Graviton could still be alive, and added "I know we see him floating in space, but he's Graviton, so you never know." When EW's interviewer then noted that they were a big fan of Talbot's transformation into Graviton, Whedon indicated that the larger fan reception to the character may yet impact his future on the show (or lack thereof):
“So if the rest of the fans feel the way you do, then chances are he might not be super dead."
The S.H.I.E.L.D. season 5 finale, 'The End', was designed to potentially serve as a series finale, which is why it leaves Graviton looking pretty lifeless as he floats away in space. At the same time, it set the stage for the now-confirmed season 6 to bring back at least one character who perished in the season 5 finale - Leopold Fitz, who was killed during the battle with Graviton - and may yet feature Phil Coulson in some capacity, even though the character was mere days (maybe weeks) away from dying by the episode's conclusion. If resurrecting players in one way or another is the order of the day for season 6, then Graviton's return from the grave wouldn't be out of place either.
On the other hand, his defeat in the season 5 finale gave Graviton/Talbot a tragic yet otherwise complete arc on the show and his return in season 6 would risk undoing that. Agents of S.H.I.E.L.D. the show finds itself in a similar predicament right now; 'The End' wrapped things up so cleanly that season 6 could come off as an afterthought or strained attempt to keep the story going. Thankfully, S.H.I.E.L.D. is a series that has continued to defy expectations and re-invent itself with each passing season, and there's fair reason to think it can do that yet again with season 6, Graviton or no.
Agents of S.H.I.E.L.D. season 6 premieres on ABC in 2019.