A slightly more far-fetched theory, suggested on Twitter by Ease editorial director Josh L. Dickey, is that Loki is posing as Bruce Banner throughout Infinity War explaining why the gamma-radiated scientist is unable to turn into the Hulk over the course of the film.
However, given the Hulk's initial bout with Thanos in the prologue, along with the recurring plot element of Banner becoming progressively more frustrated over his inability to transform into the Green Goliath in the face of overwhelming danger, we're not sold on this one. This sort of reaction would be out of place if it was Loki posing as Banner. After all, why would the trickster be frustrated in his private moments if he was faking his identity?
Perhaps the most likely theory about Loki's return isn't that the god is faking his death at all, but rather tied to the mounting evidence that Avengers 4 will significantly involve time travel. Set photos have revealed locations from previous MCU films including, most prominently, the climactic Battle of New York from The Avengers. Loki served as the principal antagonist during the attempted alien invasion of NYC and will likely reappear in sequences set during the battle as Hiddleston has been spotted on set for Avengers 4 in costume as Loki once again.
With the upcoming MCU film rumored to feature the time-traveling heroes changing history by revisiting key moments in the cinematic universe's history, perhaps Loki's death in the Infinity War prologue will be one of the elements effectively retconned away by the end of the film. This change in history would spare the trickster from Thanos' wrath and set him up for future adventures beyond Avengers 4's conclusion.