Leo Fitz has cheated death in Agents of SHIELD, potentially paving the way for the return of Ghost Rider. Over the past two seasons, Agents of SHIELD has jumped headfirst down the time travel rabbit hole, creating head-scratching paradoxes and several possible splinter timelines. One such conundrum involves the fate of SHIELD scientist, and one half of the famous FitzSimmons love story, Leo Fitz. In Agents of SHIELD season 5, Fitz was placed into a cryogenic freeze, waking up in the future to rendezvous with his colleagues and save the Earth from destruction. After achieving this goal, however, Fitz and the gang traveled back to the present-day, with the engineering genius dying soon after.
In season 6, the audience discover that a second, present-day version of Fitz is still hovering through space in ice cube form, and Simmons sets out to reclaim her back-up boyfriend. Predictably, the full implications of the time travel mechanics at play here can be interpreted in a variety of ways, but the overriding suggestion is that SHIELD prevented the Earth's destruction, causing their timeline to splinter off and thereby negating the need for Fitz to head into the future. Both theoretically and morally, the situation is shades of gray.
One entity that prefers a more "black and white" approach, however, is Ghost Rider. Introduced in season 4, the Spirit of Vengeance dwelling within Robbie Reyes has demonstrated a clear distaste for those who somehow cheat death. This is highlighted in the deal Ghost Rider makes with Phil Coulson - the Spirit temporarily bestowed its power upon Coulson in exchange for removing the alien science that was holding him together after Loki's fatal strike in The Avengers. This demand wasn't made due to any bad blood between Coulson and Ghost Rider, the Spirit is simply out to claim souls who should've already left the mortal plane.
Unfortunately for Fitz, the scientist now falls squarely into this category. While it could be argued that the laws of nature have not been broken here, since the version of Fitz that died was from a different timeline, Ghost Rider is unlikely to take such a sympathetic approach. The bottom line is that Fitz was supposed to have died, but is now parading around the galaxy as if nothing ever happened. If Ghost Rider took issue with Coulson's Kree medicine, he's surely not going to look too kindly upon multiple versions of one person existing simultaneously, and this could trigger a return for Robbie Reyes later in Agents of SHIELD season 6.
However, Ghost Rider's grievance may run far beyond just Leo Fitz. It could be argued that because the SHIELD team averted the destruction of Earth via the use of time travel, the entire planet is now guilty of defying the reaper by Ghost Rider's standards and the Spirit of Vengeance could be out for Earth as a whole. Perhaps he'll just settle for targeting the rebuilt SHIELD organization, since they're the ones responsible. In either case, a return for Ghost Rider appears very much on the cards at some point in Agents of SHIELD's future, with Reyes' story feeling distinctly unfinished.
Ghost Rider's return may have even been in motion since this season's very first episode. Agents of SHIELD season 6 has introduced Sarge, an alternate Coulson on a mission to wipe out the planet-destroying Shrikes and their creator. Exactly why Sarge does what he does, and looks exactly like Coulson while doing it, remains a mystery at present, but it's possible that this is yet another ramification of the season 4 deal with Ghost Rider - serving as some kind of intergalactic soldier to restore the natural balance. The fact that Sarge and his team use scanners that actively sweep for things that shouldn't exist on Earth (including time travelers) may add further fuel to this theory.
None of which will be any consolation to Fitz, of course, and if the Ghost Rider does come back around in the near future, he'll almost certainly have one bony eye socket on the man who should have died last season.
Agents of SHIELD season 6 continues with "Toldja" June 28th on ABC.