There's rumors that Ewan Mcgregor will make an appearance as Obi-Wan Kenobi.

This actually makes a lot of sense-as mentioned in Rogue One, Cassian says he's been fighting since he was very young, but since the Empire's been around for only about eighteen years as of this point at Cassian appears to be in his twenties or thirties, this means-and is collaborated by reference material-that he was a young soldier in the Seperatists fighting against the Republic during the Clone Wars.

This would be a perfect opportunity to have Obi-Wan show up during the Clone Wars, somehow perhaps making an impression on Cassian. Ewan hasn't aged that much physically since ROTS and of course we've already seen a decent de-aged Luke Skywalker (and this would require much less work, if any at all), so why not?