Star Wars creator George Lucas entertained the idea of a major twist involving Darth Maul and General Grievous in the prequel era, but his idea would have had major consequences and hurt the galaxy far, far away. In the Star Wars canon, Darth Maul's time as a Sith apprentice was brought to an end by Obi-Wan Kenobi in Star Wars: Episode I - The Phantom Menace. He returned in Star Wars: The Clone Wars, and his story continued through the Dark Times of the Empire's reign. It seems Lucas originally had other plans, however.
In a recent interview, Star Wars: The Clone Wars and Star Wars Rebels writer and executive producer Henry Gilroy revealed Lucas originally planned a huge prequel villain twist. He wanted Darth Maul to be brought back as General Grievous, revealed to be alive via cybernetic enhancements as the being beneath the armor. Apparently, this was an initial consideration during conversations with concept artists during the development of Star Wars: Episode III - Revenge of the Sith, though Grievous ended up becoming his own villain. While it's certainly a compelling idea on the surface, there would have been a negative impact on the Star Wars continuity going forward.
Clone Wars And Rebels Would Have Lost Their Biggest Stories
The return of Darth Maul was one of the biggest arcs in Star Wars: The Clone Wars, beginning at the end of the fourth season. Saved from exile and madness by his brother Savage Oppress and given new cybernetic legs by the Nightsisters of Dathomir, Maul was reborn and immediately began planning his revenge against Obi-Wan Kenobi and Count Dooku, the new Sith apprentice whom Darth Sidious had replaced him with after his presumed death. Maul's role during Star Wars: The Clone Wars culminated in the Siege of Mandalore in the popular and critically-acclaimed season 7, where he escaped custody during the chaos of Order 66, fleeing to form his new crime syndicate, Crimson Dawn.
In timeline order, Maul next made a brief cameo in Solo: A Star Wars Story. His narrative really continues in Star Wars Rebels, where Maul became consumed by his obsession for revenge against Obi-Wan Kenobi. His journey brought him to Tatooine where he was finally slain by the exiled Jedi Master tasked with watching over Luke Skywalker. These stories are crucial to Maul's canon history, and are very popular indeed with viewers - but couldn't have happened if Maul became General Grievous, with a direct role in Star Wars: Episode III - Revenge of the Sith. That being said, the initial inspiration can be seen with Maul receiving cybernetic legs to replace his lost limbs.
Maul Replacing Grievous Would Have Made Star Wars Feel Smaller
The Star Wars galaxy is a very big place featuring countless worlds, characters, and stories. However, there have been times when the galaxy has felt needlessly small due to a limited focus on certain projects. Luke Skywalker's complaints in the first Star Wars movie - in which he said nothing ever happened on Tatooine - are now laughable considering how often audiences are constantly being taken to the desert world. The same is true for characters, with too much focus on the Skywalkers outside the main Star Wars saga.
In that same vein, revealing General Grievous as a preexisting villain would have shrunk the scope of Star Wars as a whole. Having Grievous as his own character made for a dynamic new villain to be introduced with a unique tortured past and motivations (though he did share a strong hatred for Obi-Wan Kenobi with Maul). Additionally, Grievous being a wholly new creation opened the door for Maul to make his return in Star Wars: The Clone Wars and beyond.
Maul As Grievous Would Have Made The Rule of Two Even More Confusing
The Rule of Two - that there should only ever be two Sith at any time - was established by Darth Bane a millennia ago to preserve the power of the Sith, but Maul as Grievous would have bent it too far. Palpatine was technically already breaking the Rule of Two, corrupting Count Dooku to the dark side and calling him as a new apprentice as soon as Darth Maul died in Star Wars: Episode I - The Phantom Menace. He did the same with Anakin Skywalker, grooming him for years and ultimately having him replace Dooku. Even Dooku had agents skilled in the Force and/or lightsaber combat, such as General Grievous, Ventress, and Savage Oppress.
Maul returning as Grievous would have made things even stranger. Unless Maul's cybernetics as Grievous made him little more than a living weapon to be controlled, it would have been confusing for Sidious to have his first and second apprentices serving him simultaneously. This would have been even odder given the clear parallels between the Maul/Grievous mash-up and Darth Vader, a Sith apprentice crippled and saved through cybernetics in the same fashion.
Palpatine’s Vader Plans Would Have Been Even More Complicated
The idea of Maul as Grievous would have also made Anakin Skywalker's corruption into Darth Vader more complex. Killing Count Dooku at Palpatines' urging was a major step in Anakin's corruption, making him unbalanced and more susceptible to the power of the dark side. However, the assumption would be that a Maul/Grievous hybrid would have had to be dealt with as a second Sith apprentice, either by Anakin himself or by Obi-Wan Kenobi who canonically slew Grievous on Utapau.
If Kenobi was the one to defeat Maul as a cyborg, there would have been some lost agency for Anakin inheriting his new role as Palpatine's apprentice. Conversely, Anakin killing Maul would have taken away from Obi-Wan's established rivalries with both Maul and Grievous, regardless of whether Maul was the one beneath the armor or not. Likewise, General Grievous was always meant to be a precursor to Skywalker and his own cybernetics as Darth Vader. Adding a secret identity reveal as Darth Maul to the Star Wars continuity would have likely felt like a significant retread of Darth Vader revealing himself as Luke's father in The Empire Strikes Back. Lucas always said Star Wars should be like poetry, but some rhymes don't work.