Warning: Contains SPOILERS for Sonic the Hedgehog 2!
Sonic the Hedgehog 3. So, while Shadow is already a huge tease in Sonic 2, the film might've shown its hand with GUN, hinting at key elements of the plot of Sonic 3.
's post-credits scene shows two of GUN talking about mysterious coordinates they have found that lead to an abandoned science project from 50 years ago. It is soon revealed that this was none other than Project Shadow, the experiment that lead to the creation of the ultimate life form, Shadow the Hedgehog. Shadow is a popular antihero from the Sonic game series, having a surprisingly dark and fleshed-out backstory.
In the games, Shadow the Hedgehog was created by Professor Gerald Robotnik, the grandfather of Dr. Eggman. Robotnik started Project Shadow in an attempt to discover immortality so that he could save his granddaughter Maria who had a terminal illness. Although the cure didn't work, it produced Shadow, who became close friends with Maria while the two were confined on the space station where the experiment took place. Later, GUN raided the site in an attempt to shut down the project, and Shadow the Hedgehog's powers weren't enough to stop them. Maria successfully helped Shadow escape, but in the process, she was shot and killed by GUN soldiers. The death of Shadow's young friend had a massive impact on him, sending him down a very dark path. While this may seem heavy for a franchise about speedy hedgehogs, it is very effective, and the introduction of GUN and Project Shadow hints that Sonic 3 may explore this backstory.
Sonic 2 shows that GUN has an interest in taking down alien lifeforms, as they do with Sonic in the film. Since they are already aware of Project Shadow, it is likely that they had a hand in shutting it down 50 years ago. There will be some changes, as they talk about the location of Project Shadow's research facility, and they make no reference to it being in space. Also, the files regarding Project Shadow were found at the as Dr. Eggman was being erased from GUN's database. It is likely that it is tied to Eggman because his grandfather, Gerald Robotnik, was the mastermind behind the project. Furthermore, GUN is headed by Commander Walters in the movies, a character who is old enough to have plausibly been involved with Project Shadow's shutdown.
Having a significant chunk of Sonic 3 take place 50 years ago is unlikely, but Shadow's backstory could easily be told through flashbacks or old recordings. The Sonic games even have time-traveling characters, so that is another possibility. Either way, with the appearance of GUN, Shadow, and hints at Robotnik's ties to the project, it seems that Sonic the Hedgehog 2 is setting up a version of Shadow's tragic backstory.
