Star Wars: The Rise of Skywalker's original MacGuffin was a kill switch that wiped out all First Order tech. With the film marking the definitive end of the Skywalker saga, director J.J. Abrams and co-writer Chris Terrio had to come up with an ending in which the Resistance defeated their enemies once and for all, leaving no doubt evil had been toppled. There's no denying that was a tough task, particularly since everyone thought the galaxy was saved after Return of the Jedi, only for the First Order to emerge decades later and wreak havoc.
Bringing Emperor Palpatine back to the fold, the creative team opted to give the dark lord a massive fleet on Exegol, which he planned to use to rule the galaxy with an iron fist. Ships could get to Exegol through use of a Wayfinder, so The Rise of Skywalker became about the hunt for the Wayfinders (only two were made) so the heroes could fly to Exegol and wipe out Palpatine's army. However, initially things were quite different and there was another MacGuffin as the focus of the movie.
In the book The Art of Star Wars: The Rise of Skywalker, prop concept designer Matthew Savage revealed at one point in the script's development, the MacGuffin was a "kill switch for all First Order tech, constructed over many generations by an Empire-hating race." You can see concept art of what was referred to as the "sabotage device" in the space below:
It's unclear why the kill switch idea was abandoned during the process, but had it stuck, this would have been where Babu Frik came in. The fan-favorite droidsmith was originally part of the "Empire-hating race" that was small enough to clean Star Destroyer engines and developed the sabotage device throughout the years. The main group of Resistance heroes would have to meet Babu to collect the "nearly complete" object and go about their mission. However, Terrio felt this angle wasn't "right for this movie" and they went in a different direction. A case can be made The Rise of Skywalker was better for it, since even by Star Wars standards, a magic switch that wipes out First Order tech reads as a little too convenient.
Still, it would have been interesting to learn more about Babu Frik's species and everything they went through. In all likelihood, Babu's kind wasn't treated very well by the Empire and the First Order, serving as another illustration of the villains' tyranny over the entire galaxy. Not that the movies needed more proof for why audiences should root against the antagonists, but this could have been a poignant angle that would have made Babu a little more than just an adorable scene-stealer. But as it stands, Babu was one of the more universally praised aspects in The Rise of Skywalker, so it all worked out in the end.
Source: The Art of Star Wars: The Rise of Skywalker