The version of Loki from the main Marvel Cinematic Universe timeline is dead, having been brutally murdered by Thanos at the start of 2018's Avengers: Infinity War. Therefore, the Loki series on Disney+ will feature the version of the character who escaped in 2012 when the Avengers went back in time to obtain the Space, Mind and Time Infinity Stones from New York.
By escaping the way he did, Loki broke the original timeline, creating a new one. Given the amount of power being thrown around in the MCU multiverse, timeline corruption has probably happened on countless occasions. A Reddit theory that suggests the Loki series will see the titular God of Mischief having made a deal with the TVA (Time Variance Authority) to repair or close broken timelines by committing crimes, heists or assassinations in exchange for ultimately being returned to his own.
But the theory comes with an additional, tantalizing hook. It suggests that Loki will attempt to secretly acquire items of power during his missions in these alternate timelines, such as the Infinity Stones. Having recently possessed both the Mind and Space Stones, this version of the character would surely want to get them back. And, like the Avengers in Endgame, he'd have all of history to play with when it comes to choosing times and places to pluck them from. It could lead to intriguing confrontations with characters like the Red Skull, the Nova Corps, Goose and/or Star-Lord. He could even attempt to steal the Mind and Space Stones from alternate versions of himself, which could explain the possible appearance of Lady Loki.
Amusingly, this could lead to a running gag in the series, whereby the TVA pull him back to their headquarters and foil him every time he gets close to acquiring a powerful object. It could be the case that, in the trailer, Loki jumping out of the airplane as D.B. Cooper was his attempt to escape the watchful eyes of the TVA, only to be recaptured by the organization before he succeeds. This would in turn serve as the in-universe explanation for D.B. Cooper's disappearance in the real world.
Ultimately, the theory suggests that this will all lead to Loki rejoining the primary MCU timeline, enabling him to seamlessly appear in the franchise's movies again. That would open up a fascinating new arc with Thor, who had previously made peace with his brother, only to watch him die at the Mad Titan's hands. This version of Loki will have none of his previously developed affection for Thor and could go on to be a heartbreaking and formidable enemy to the God of Thunder. It could even be Loki who unleashes Gorr the God Butcher in 2022's Thor: Love and Thunder.