Just like the first Sonic the Hedgehog movie did before it, Sonic 2 leaves viewers with a taste of what's to come next, as its post-credits scene reveals Shadow the Hedgehog. This gives fans plenty of ideas as to what the next movie will deal with, and for fans of Sonic Adventure 2, this teaser is rather exciting. However, there's a chance that the third Sonic movie also marks the start of the cinematic universe Paramount is building leaning into lesser-liked parts of Sonic history.
While Shadow was introduced in Sonic Adventure 2, the Sonic the Hedgehog franchise has built up more lore about Shadow's creation and purpose since. The biggest example of this is in the form of Shadow's solo game, Shadow the Hedgehog, which reveals that the black hedgehog had much more go into him than previously thought. However, while this is just a fact when it comes to how Shadow was made, it comes from a title that many consider odd, and including it in the movies could damage the reputation the cinematic universe has built already.
Sonic the Hedgehog 3 is Sonic Adventure 2
It's understandable why people would think the third movie would deal with the plot of Sonic the Hedgehog 3 & Knuckles, but Sonic 2 already adapted the plot of the classic Sega Genesis title. Instead, the post-credits scene in Sonic 2 makes it clear that the story will most likely deal with the plot of Sonic Adventure 2, as "Project Shadow" is discussed in detail. Project Shadow was the government-funded experiment to create the ultimate life form, but was shut down under the idea that a "horrible accident" had occurred during research.
Adapting Sonic Adventure 2 into a movie sounds great. Sonic Adventure 2 is not only considered to be one of the greatest Sonic games of all time by a majority of the series' fans, but the game features a lengthy plot that matches the length of a feature film rather well. The game not only introduced Shadow, but also Rouge, and these characters were very important in setting the stage for what came next for the series. Shadow's backstory in particular went farther than just SA2, and Shadow's movie debut may want to use this to its advantage.
Shadow the Hedgehog and the Black Arms
In Shadow the Hedgehog, the titular hedgehog learned a dark secret about himself - he was created with the help of Black Doom, the leader of an alien race known as the Black Arms. Professor Gerald Robotnik, Shadow's creator, had made a deal with Black Doom to create the hedgehog using the alien's DNA. In return, Earth would belong to Black Doom once the nomadic race that lived on a comet came back in 50 years. This led Gerald to create the Eclipse Cannon, making sure Shadow could be the one to destroy the Black Arms upon their return.
This deal is what caused GUN to storm the Space Colony ARK to shut down the project in a raid that caused the death of Gerald's granddaughter, Maria. Fueled by revenge, Gerald remotely altered Shadow's memories to have his purpose changed. Shadow was then found by Dr. Eggman and Sonic Adventure 2 began. In his solo game, Shadow remembered what he was truly made for as part of the true ending. Due to how SA2 was made before Shadow the Hedgehog, the third Sonic movie won't necessarily need to include the details found in the spin-off title, but it may make the other two films flow better with the third if it does.
Including Black Doom in Sonic 3
The first Sonic movie dealt with how Sonic was remaining hidden on Earth to protect his power from falling into the wrong hands. He was an alien, and the humans in the film reacted to him as such. Many people on Earth had no idea a creature like Sonic existed, yet Shadow, a hedgehog just like Sonic, was created 50 years ago by the government. While this keeps Shadow's backstory true to Sonic Adventure 2, it leaves viewers with the question of just how that came to be when people on Earth had no idea creatures like Sonic and Shadow existed. This is where Black Doom could come into play.
Black Doom would fit rather well with the narrative the Sonic cinematic universe has built so far. The Black Arms travel across the galaxy on a comet, circling Earth's orbit. Black Doom could have DNA from an old conquest from Sonic's world, handing it off to Gerald Robotnik in exchange for Earth. With how Sonic, Tails, and Knuckles are already seen as aliens from another world, including other aliens wouldn't feel out of place. However, the big problem is that Shadow the Hedgehog is seen as one of the stranger games in Sonic history, and that might hurt Sonic 3 as a result.
Shadow the Hedgehog is considered by many fans to be one of Sonic's "dark ages" titles. These games didn't have the best positive press when released, and even though some fans have accepted the role the game plays in Shadow's backstory, others might not be so excited to have such a poorly received title be adapted into the cinematic universe. Those who aren't fond of Shadow the Hedgehog believe the addition of the Black Arms to Sonic lore is rather silly, which may hurt ticket sales as some fans lose interest.
However, there's a chance that the third Sonic movie might actually benefit from including Black Doom instead. The Black Arms could just be seen as other lifeforms in the galaxy, similar to Sonic and his friends, and that might lend a hand in fixing the confusing question of how Shadow existed before Sonic had even reached Earth. Black Doom could also serve to be a surprise antagonist for the film in the absence of Jim Carrey, since the actor has spoken about possibly retiring after Sonic the Hedgehog 2. No matter which direction Sonic the Hedgehog 3 takes with Shadow, fans of both the movies and the games are certainly in for an interesting third movie.