The most famous scene of ROGUE ONE was totally a last minute idea..

Gareth Edwards in an interview with Fandango:

He arrives and obliterates the Calamari ship, and then the blockade runner gets out just in time and he pursues the blockade runner. And then [editor] Jabez [Olssen] was like, ‘I think we need to get Darth on that ship,’ and I thought, yeah, that’s a brilliant idea and would love to do it, but there’s no way they’re going to let us do it. It’s a big number and we had, what, like three or four months before release. Kathy [Kennedy] came in and Jabez thought, fuck it, and pitched her this idea, and she loved it. Suddenly within a week or two, we were at Pinewood shooting that scene.