Lucas did intend to make the prequel trilogy while producing the OT but a combination of his messy divorce and feeling the special effects tech wasn't quite there yet delayed him starting it; however he was impressed by "Jurassic Park" and restarted the prequel film trilogy almost immediately after, in 1994, originally for a 1997 release but Lucas decided for the special editions that year instead (although production did start that year).
The idea of a sequel trilogy (or even a fourth trilogy with parts 10-12) seems to have been largely abandoned when Gary Kurtz left, and Lucas seemed to have little interest in making it until fairly close to the Disney sale where it seemed he did some preliminary work and discussions with Mark/Carrie at least, but ultimately decided to retire and 'pass the torch' to Kennedy and new creators (Although this also included some of his colleagues like Knoll, Filoni and Howard).
However, what if these didn't happen and say we got, episodes one I-III in 1986-1992, followed by a sequel trilogy in 95-2001 or thereabouts (or the other way around?) what would the cast be like? Would the story have been different without the CG effects etc?