NOTE: This is speculation, some might say wishful thinking. I?m not siting sources, just offering an opinion.
(UPDATED BELOW) Star Wars movies have shrouds of secrecy around them, it?s a given. It isn?t just JJ Abrams who keeps things close to his chest. One of the few tidbits that has leaked about the next Star Wars movie, Rogue One, is that there will be a few appearances by characters that we are familiar with. So far (March 14, 2016), the cast is named, but the characters aren?t.
Much of his story was axed from Star Wars, fairly late in the film?s development (his scenes had music scored for them already – that?s a late stage process).
What do we know about Biggs? He was from Tatooine. He was Luke?s best friend. He went to the Academy, the same one to which Luke kept deferring his application.
From deleted scenes we learn that the Academy isn?t exactly Imperial service, but you can be drafted from the Academy to serve in the Imperial military. Biggs is apparently serving on a ship, although it is implied that he is still not a graduate of the academy, so perhaps on some kind of work term/internship. He intends to ?jump ship? and join the rebellion.
Later we learned that he is a trusted pilot in the rebellion. He, unfortunately, is one of the casualties in rebel?s defense of Yavin.
Could Biggs be directly involved in stealing the Death Star plans? No, probably not. The timing wouldn’t work out ? unless he was back on Tatooine as part of a ‘chase’ team that was shadowing the plans’ voyage to ensure that they arrived safely… but probably not.
Could he be peripherally involved in some Academy plotting that leads the rogue one team to the plans? Absolutely. That could be why he?s confident that he can join the rebellion. It might also be why he is considered a trusted pilot within the rebellion in such a short span of time.
Just my thoughts. Call me crazy.
UPDATE: It turns out i’m not the only one to have had that thought. SlashFilm considered the same idea when the first cast photo was released last year. Diego Luna looks a lot likeGarrick Hagan did when he played Biggs. Apparently Twitter was all over it too: