Stealing the plans for the first Death Star, almost was a moment we saw on TV. Speaking to Comic Book, Dave Filoni revealed that early discussions regarding Rebels included what eventually became the plot of Rogue One: A Star Wars Story.
There were some really early on conversations with Rebels about it being about them finding the Death Star plans. But as Rogue One came about and took shape, it was obvious we weren’t gonna do that story, I didn’t wanna do that story, frankly, honestly, with Rebels because I thought I don’t want their whole existence to just serve and hand off to another part of the plot. I want it to be its own story about this kid.
But Rebels did crossover with Rogue One. Chopper made a brief cameo, Hera Syndulla got name-dropped, and her ship, the Ghost, was in the movie too.
Rebels, in turn, returned the favor. It brought in Rogue One (and original The Clone Wars) character Saw Gerrera, and also teased the Death Star.