One thing we know is that Rogue One: A Star Wars Story did a lot of filming that didn't end up in the final film. However, fans hoping for an "extended cut" on DVD will likely keep waiting. According to the editors who put together Rogue One, the first cut of the film, which tends to have almost everything that works included in it, was barely longer than the final cut that we saw in theaters.
[The assembly] was not much longer than the finished film. I think the first assembly was not far off actual release length. Maybe 10 minutes longer? I genuinely can't remember because that was nearly a year ago now. There's no mythical four hour cut, it doesn't exist.
Traditionally, the first thing the editor does is assemble everything together into one long movie. It usually includes one take of every scene that was shot for the film. Most of the time there's never any intention to release the film that way, though it would seem that, as far as length goes, they could have done that with Rogue One: A Star Wars Story, and it would have worked out fine.
Three different editors worked on Rogue One: A Star Wars Story. Jabez Olssen and Colin Goudie worked on the film from the beginning, and John Gilroy was brought in, apparently during the reshoots to get a fresh set of eyes on the process. However, once those reshoots were done, Goudie tells Yahoo Movies UK that there wasn't actually a lot of extra footage. While we may see some additional scenes included in streaming and Blu-Ray releases, it would seem that we might barely notice the additions.
Of course, part of the reason that there may have been very little added to the assembly was that some portion of the film went through some significant changes. Rogue One saw reshoots that, according to sources, significantly changed the third act of the film. The editors also talk about how many of the scenes the first introduce us to some of the team, were also added during the reshoots.
It seems that because the reshoots were done, they had a specific plan about what they needed to create in order to make the movie work in the way that they wanted. For that reason, there simply isn't a lot of additional footage that work in an extended cut of the movie. Everything that's left over is from the old version that just got replaced.
In the end, Rogue One: A Star Wars Story probably doesn't need an extended edition, the movie isn't missing anything that it needs. Now an alternate version, that's another story.