Following the death of Jigsaw, Amanda Young should have become the main villain of the Saw movies. Saw was the brainchild of writer Leigh Whannell and director James Wan, who were so desperate to make a movie they wrote one that could be shot for a tiny budget. That's where the core idea of two characters locked together in a bathroom came from, but they later managed to sell the script and producers attached actors like Cary Elwes and Danny Glover to the movie. Saw became a shock success back in 2004, with the film being a taut, cleverly constructed thriller with a stunning twist ending.
Saw was never supposed to launch a series, but not only has it received eight sequels - including Chris Rock's Spiral - but it spawned a whole subgenre. The original features little in the way of genuine gore, but its gruesome traps and focus on suffering would give rise to movies like Hostel or Captivity that focused on torture. This movement is generally critically dismissed by critics, though the likes of the first Saw or even New French Extremity movies like Inside are still well-regarded.
The original Saw introduced Amanda Young (Shawnee Smith), a reformed drug addict who is forced by Jigsaw (AKA John Kramer) to undergo the famous "Reverse Bear Trap" test. She barely escapes with her life but claims Jigsaw's test helped her, though she's later pulled back for Saw II. The twist of that sequel revealed she became one of Kramer's (played by Tobin Bell, who almost had a bizarre Spiral cameo) apprentices, but while he planned for her to succeed him after he passed away from his cancer, she failed his final test and died in Saw III. However, the series missed a trick by not making her the new villain.
In a sense, Saw III would have been the perfect ending to the saga. It closes with Kramer and Amanda dead - but even in death, Jigsaw still had his revenge on Saw III's "hero" Jeff. Corrupt Detective Hoffman (Costas Mandylor) was another of John's secret students and took over following his demise, but he lacked Kramer's twisted "morality" and was a mere serial killer. Sadly, Hoffman lacked the charisma and gravitas of Bell's Jigsaw, and given the path the character goes down, the series should have just made Amanda Young Saw's new villain.
Saw's Amanda Young had a close, emotional connection to John, though it was clear she didn't share his moral code. It would have been much more interesting to see her embark on a self-destructive journey as the new Jigsaw and struggle to balance the weight of honoring Kramer's memory with her own twisted (sometimes literally) sense of justice. Having her pair off against Elwes Dr. Gordon in Saw 3D - the original "finale" to the series - would have also made for a good, circular arc. Smith's Amanda Young is one of the best-developed characters in the Saw franchise, and having a female horror icon fronting the series would have been a smart move. For better or worse, it was decided Saw III was the end of her journey - but it feels like there was still more story left to tell with Amanda.