Played by Clark Gregg, S.H.I.E.L.D. Agent Phil Coulson played a central role in Phase One of the Marvel Cinematic Universe, serving to tie together several of those early films. What's more, his apparent death at the hands of Loki in 2012's The Avengers was a catalyst for uniting Earth's Mightiest Heroes. Now, a new fan theory suggests he may once again be pivotal to the events of the movie universe.
Coulson's demise was, of course, only temporary, as he was brought back for the ABC drama Marvel's Agents of S.H.I.E.L.D., which spent its first season explaining his revival through Kree technology. Agents of S.H.I.E.L.D. has run for five seasons, and is returning for a sixth next summer. Interestingly, while Coulson stepped down as S.H.I.E.L.D.'s director to face his impending death in the Season 5 finale, he's listed in the cast for Season 6 -- but not for Season 7, which has already been announced.
In the meantime, fans have been hunting for clues about how the events of Avengers 4 will play out when it hits theaters in May. Details have been kept under wraps, but it seems likely that time travel will be involved, which means that any and all MCU characters have the potential to make an appearance. The theory presented on Reddit points out that MCU film scripts are written far in advance of the television shows, and that the delay in the Season 6 premiere of Agents of S.H.I.EL.D. conveniently places it after the release of Avengers 4. Storylines that intersect with the MCU's movies have always been a feature of Agents of S.H.I.E.L.D., but the chronological setting of Season 5 meant that it didn't address the shocking ending of Avengers: Infinity War, so it could easily pick up again with its own interpretation of events once they've been resolved in the movies.
A prophecy issued in Season 5 has indicated Coulson as the one who will "bring the pieces together," which would be an apt function for him if he were to appear in Avengers 4. Either way, he'll be seen on the big screen again in the 1990s-set Captain Marvel, coming on March 8.
Returning in July 2019 on ABC for its sixth season, Agents of S.H.I.E.L.D. stars Ming-Na Wen, Chloe Bennet, Henry Simmons, Iain De Caestecker, Natalia Cordova-Buckley and Elizabeth Henstridge.