Continuity is a big thing when it comes to large stories. Because if things don’t click together like they should, or things are blatantly contradictory, people are going to notice. This was a problem for Star Wars for some time until the “new universe” was made, and certain parts of Star Wars lore, called the “Extended Universe”, were cut out. Thankfully, the Clone Wars cartoon was kept in canon. Thus, it allowed for Saw Gerrera to come into Rogue One, and possibly a lot more.
Saw Gerrera was first made in the Clone Wars cartoon, as a completely original character. Not surprisingly, he was a freedom fighter that came into contact with many big Star Wars names. Including Anakin Skywalker, Ashoka Tano, and Obi-Wan Kenobi. So when the time to make Rogue One came, and they had an opportunity to bring him back, they did.
What makes this interesting though is that Gerrera is set to potentially have a big future in the Star Wars universe. He’s already scheduled to return via Star Wars Rebels, and will even be voiced again by Forest Whittaker. But, in an interview with Empire, Lucasfilm President Kathleen Kennedy stated that the two-part return may not be the end.
According to her, they wanted to bring back Saw in a big way, but because of how the film was cut, they didn’t get to delve as deeply into him as they wanted. So, she notes that it’s very open with what happens to him in the future.
Likely, this means more Star Wars Rebels appearance, but, it could be something else. Maybe a comic book by Marvel, where the entirely Star Wars line is currently, or something else entirely! Time will tell where “the original Rebel” will come up next.