Since Rogue One deals with the Galactic Civil War, there’s always the possibility that Emperor Palpatine shows up to deal with a situation.
If Gareth Edwards was planing on incorporating the Emperor, chances are McDiarmid (who remains enthusiastic about Star Wars) would have come back.
Rogue One: A Star Wars Story opens in U.S. theaters on December 16, 2016, followed by Star Wars: Episode VIII on December 15, 2017, the Han Solo Star Wars Anthology film on May 25, 2018, Star Wars: Episode IX in 2019, and the third Star Wars Anthology film in 2020.
“Rogue One’s on its way, when I was in charge of things – I mean the Emperor, not me.