The place to stick another trilogy is before the trilogy before the OT so Star Wars (1977) would be episode 7.
That way you could show a truly primitive version of the galaxy in a brutal war between Sith warlords and Jedi knights leading to the formation of the Republic.
A thousand generations later. After a chance meeting with Obi-Wan, Anakin becomes fascinated by tales of the Jedi and leaves behind his wife and brother to join the order. He seems to be killed in the Clone Wars. Darth Vader hunts down and destroys the Jedi. Anakin's wife hides on Alderaan while Owen raises Anakin's son on the distant world of Tatooine.
Star Wars, the Empire Strikes Back remain the same.
Revenge of the Jedi, Civil unrest is on the rise. Luke finishes his training with Yoda. Han is rescued from the Hutt palace on a gambling/gladiatorial pleasure planet. The Emperor building a fleet of Death Stars uses Vader as a trap to capture Luke as a replacement. Vader sacrifices himself is redeemed and tells Luke about his twin sister.
The final trilogy. The civil uprising has been repressed by every world being in range of a Death Star. Luke and the Empire both search for Luke's sister who has been declared the Imperial heir by the aging dictator. The two siblings first fight and then join forces to close down the network of battlestations as the Rebel and Imperial fleets clash.