I know a lot of people feel making Rogue One about stealing the Deathstar plans was a bad idea.
Thing is I just re-watched Episode IV, and the opening scroll says that the Rebels just won their first victory against The Empire and that the Deathstar plans were stolen at the same time. So really if you want to depict a pre-Yavin Rebel victory that battle is the only option. Even Rebels doesn't contradict that, it's not really depicting any actual proper battles.
Now I know some people don't like spin off films being more Prequels at all, they wish they were set in the same era as the new films. I couldn't feel more the opposite. I don't want to know anything about what happens after Episode VII till we see Episode VIII. Star Wars trying to do something like the Marvel Cinematic Universe would be a mistake.
The spin off films are for now better off staying in the era fans already care about, the new era will get it's EU material once it's proven itself in it's actual Episodes. But it would be a mistake for Disney to just presume their film will be a success before it even comes out.
Rogue One is probably going to be about the battle, and the spies getting the Deahtstar plans a subplot that may even be mostly off screen. Rouge One will come out about when Rebels Season 3 might end, I wonder if it's possible they will make a lose tie in there?
I really hope Jimmy Smitts has a cameo in the film as Senator Organa. As well as Admiral Akbar.
And as a nod to Rebels just show The Ghost, don't try to write it's characters in but just showing The Ghost as part of a Rebel fleet would be a fun nod with no risk of continuity lock out for people who don't follow the show.