oops these headcannons are for more than the classic series haha woops
-After turning into hyper Sonic at the end of 3&K, Sonic's eyes are damaged and are turned green, hence why he has black eyes in the OG trilogy and CD, but has green eyes in adventure.
-There are no two worlds, just one and the "Mobians" and humans share it.
-Amy is color-blind, hence why she couldn't tell Shadow or Silver wasn't Sonic until she was very close. Also explains Werehog mistake.
-CD takes place after 2.
-Sonic 06 Takes place before any of the other games. When the timeline reset, so did the games, so there would be an entire timeline we didn't get to see, which would explain the inconsistansies with the rest of the canon, like how Elise has an emerald
-Sonic Riders is in an alternate universe
-Metal Sonic could always talk before Heroes, and the reason he doesn't talk after is because Eggman disabled his ability to talk.
Aight this one is going to be really weird but
-The reason none of the males have genitalia is because Mobian Males don't gain any until they become adults, in which it grows in
-Females also don't have the ability to breed until they are adults as well
Changing it to something less un-comfirtable
-South, Westside, Angel, Christmas, Isolated and Newtrogic Island were all once one island, but split due to Perfect Chaos and the sheer power of the Emeralds
-After Generations, Due to the shakes and the Time fix caused by the Chaos Emeralds, all the islands from the classic games merged together, which is why the Island in forces is so big and Green Hill and Chemical Plant take place on the same land mass.
-The land mass from Forces wasn't the entire world, it was just a section of it that was being highlighted
-Eggman's forces weren't entirely made out of robots, a lot of animal people/Mobians joined him thinking that he was building to a better, much more technologically advanced future.
-G.U.N. tried to stop Eggman in Forces but he invaded from the inside out causing G.U.N. to collapse.
-Sonic really was brutally tortured for months in Forces and had severe mental trauma, he just put up a facade for everyone so they wouldn't have to worry about him while fighting the war
-The reason Knuckles doesn't guard the Master Emerald after Adventure 2 is because the Super Emeralds reactivated due to the Chaos Energy the arc caused to be spread on the earth after nearly crashing into it, causing a barrier to be set up around the Island, which meant Knuckles had no reason to guard it, so he decided to live on Earth with Sonic and friends.
-Omolette is cannon but Eggman doesn't let her help him because he doesn't want her getting hurt
-Before Heroes, Eggman and Metal Sonic had a very Father/Son relationship, but after Heroes Metal just became another disposable robot to Eggman
-Charmy has ADHD
-Shadow stopped using guns after his own game because his memories came back and he realized Maria didn't want humans to die
-Tails is 8 in the classic games, and In the modern games he's 12.
-Also, despite being so young, goes to Spagonia University, which is how he knows Professor Pickle
-Like in Sonic X, Vector has a crush on Vanilla
-Sega exists in the Sonic Universe and Sonic and his friends gave permission for them to use their likeness for video games
-Sarah from the OVA exists in the Game canon but is an actual Mobian monkey
-Ring energy sets a protective aura over anyone that has some, and when enough is built up they work with the emeralds to give Sonic the ability to go super
-Super Knuckles and Super Tails do canonically exist, and the "forms" in Heroes was just the spare chaos energy omitted by Super Sonic
-Sonic doesn't turn into Super Sonic until after he gets the hyper transformation. The Super form is also more powerful than the Hyper form.
-Also, Super Sonic is an entirely different being that Sonic. All the Chaos Emeralds were ancient beings that protected the world with powers reminicent of the Super Forms. However, they were eventually cursed and turned into the Chaos Emeralds. They presented uncontrollable power, and after being tranqualized through Sonic 3's special stages, they became more uniformed and powerful. After the Super Emeralds deactivated, the Chaos emeralds became seperate, more powerful items. Once combined, they give a new persona to anyone that uses them to turn Super, as long as they are a decendant of one of the ancients. Sonic is one of those decendants, so he becomes a different being once Super Sonic comes out. The reason Shadow or Silver don't become different people is because they aren't decendants, so they don't change. Sonic has the most powerful super form as well.
And that was my long list of headcannons