2) A New Hope Unused Footage
During the triumphant final act of Rogue One, the band of renegades are helped out by the Rebel Alliance fleet in all its glory. To complete the harking back to A New Hope, deleted footage from that movie is spliced in. Most notably, two characters from the original movie, Gold Leader and Red Leader (as played by Drew Henley and Angus McInnes), are among the fleet.
Another easily-missed connection between the two movies is the callsign Red Five. One pilot going under the sign was killed in the battle over Scarif. This then leaves the callsign vacant, ready for Luke to take it up when he joins the Rebels in A New Hope.
Similarly, Rogue One reaffirms that women are among the Rebel pilots. There were originally three female pilots in A New Hope but all were edited out of the final cut. One of Rogue One’s female pilots is voiced by Star Wars Rebels actress Vanessa Marshall.
Speaking of which…