It’s noteworthy that Ringworld almost wasn’t written, as Niven was thinking that he’d soon have run out of things to write in that setting. Norman Spinrad suggested that he should write a final novel that basically retconned the whole background of the series. “Then why don’t you write a novel that tears it to shreds? Don’t just abandon known space. Destroy it!” He’s published the outline of that novel, Down in Flames, in his collection N-Space. In the end he wrote Ringworld instead, which contradicted the planned novel.
Some of the ideas from the outline appear in the second half of the novel Protector, the first half of which introduced the species that were retconned to be the Ringworld Engineers. A later novel of the series actually did retcon a basic part of the setting, changing the reason why ships in hyperdrive disappeared if they got too close to a star.
Edit: Down in Flames is available online, for those who are interested.