Rogue One will probably be on blu-ray in April. Releasing Star Wars in a 40th Anniversary edition in May would be great cross promotion. You have the film that Rogue One leads to. You have the original version, not seen in years and you can stick a trailer for Episode 8 on it to add to the excitement. It would be a win.
I think there is zero chance they are going to cannibalize sales of RO with some other major release.