Once you defeat Spoiler:the Supreme Titan on Easy and Normal difficulties, the game's final boss Spoiler:"The End" will end up being defeated automatically in the final cutscene. However if you defeat Spoiler:the Supreme Titan on Hard difficulty, you will actually have a boss fight with Spoiler:The End before it's defeated in the cutscene.

Both endings are essentially the same except for said boss fight, and the ending song either being "Vandalize" by ONE OK ROCK on Easy or Normal, or "One Way Dream" by Nathan Sharp on Hard.
Contributed by SOGESNAKE
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In rare instances throughout the game, characters that didn't appear in Sonic Frontiers are referenced in passing, such as Cream, Shadow, Rouge, Blaze, Jet, Omega, Yacker the Wisp, and Zavok of the Deadly Six. However two of these voice lines reference Sticks the Badger and Tangle the Lemur, two characters who have not appeared in the mainline game series at all. While Sticks previously had a character page among other mainline characters on the Japanese Sonic Channel website, she has so far only appeared in the spin-off series Sonic Boom, while Tangle has so far only appeared in IDW Publishing's Sonic the Hedgehog comic book series. Thus Sonic's name drop of them in this mainline title essentially establishes the characters as canon to the Sonic game series. This Easter egg may have come about due to the fact that Sonic Frontiers' writer Ian Flynn is also a co-writer of the IDW comic series.
Contributed by PirateGoofy
Spoiler:The beings known as The Ancients that are seen in flashbacks are confirmed by Egg Memo 11 to be closely related to Chaos, the God of Destruction, the main antagonist of Sonic Adventure.
Contributed by PirateGoofy
Some of the Cyberspace levels have layouts and elements taken almost directly from previous Sonic games, even if the aesthetics of the Cyberspace levels don't match the legacy ones at all sometimes. They are as follows:

Zone 1-1: Windmill Isle Act 1 from Sonic Unleashed

Zone 1-2: Windmill Isle Act 2 from Sonic Unleashed

Zone 1-4: Green Hill Act 2 from Sonic Generations

Zone 1-5: Chemical Plant Act 2 from Sonic Generations

Zone 1-6: Green Hill Act 1 from Sonic Generations

Zone 1-7: City Escape from Sonic Adventure 2

Zone 2-1: Green Hill Act 2 from Sonic Generations (2D sections)

Zone 2-2: Dragon Road Act 3 from Sonic Unleashed

Zone 2-4: Shadow's Radical Highway level from Sonic Adventure 2

Zone 2-5: Sky Sanctuary Act 2 from Sonic Generations (2D sections)

Zone 2-6: Shadow's Sky Rail level from Sonic Adventure 2

Zone 2-7: Sky Sanctuary Act 1 from Sonic Generations

Zone 3-1: Green Forest from Sonic Adventure 2

Zone 3-2: Savanna Citadel Act 1 from Sonic Unleashed

Zone 3-3: Sky Sanctuary Act 2 from Sonic Generations (3D sections)

Zone 3-6: Rooftop Run Act 2 from Sonic Unleashed

Zone 3-7: Chemical Plant Act 2 from Sonic Generations (2D sections)

Zone 4-1: Metal Harbor from Sonic Adventure 2

Zone 4-6: Chemical Plant Act 1 from Sonic Generations
Contributed by PirateGoofy
Development began shortly following the release of Sonic Forces, which drew some criticism for its extremely short length and level design. Hearing this criticism, Sonic Team decided to abandon the development method used since Sonic Unleashed, since they feared the next game wouldn't satisfy fans.

Generally, 3D Sonic games often limited and constrained players from roaming around the scenery, so they decided to make the game completely open-world, with the idea of innovation that hadn't been seen since the original game and Sonic Adventure.
Contributed by GamerBen144