The main intention of Sonic Forces was to bring the Sonic series back to it's more serious outings since the original introduction of Sonic's Modern redesign as opposed to the more casual & pop-oriented games at the time starting with Sonic Colors in 2010. The game's art director Miura Yoshitaka stated: "We wanted to achieve not only realism, but also the atmosphere of the design culture that existed in 80s graphics in 3D." In order to make these basic concepts compatible with the look of Sonic's world, they chose to work on an improved version of the Hedgehog Engine previously used for Sonic Unleashed and Sonic Generations called the Hedgehog Engine 2, which was built in-house and combined partition boot record-based image creation with character-driven expressions that are not simply focused on realism.

Both the game and the Hedgehog Engine 2 were developed at the same time over the course of three years, and because of this simultaneous development, the designers went through trial and error trying to adapt to the changing workflow for the first year until the engine was eventually optimized well-enough to complete production of the game at an easier scale for the smaller number of people working on it at the time.
Contributed by SkyminHAZBOZ
In the PC version, there are several unused animations stored in the game’s files. These include animations carried over from Sonic Generations for Classic Sonic, a goal post animation from Generations, a "seek" motion for The Egg Pawns, a damage animation for Silver, early boss fight animations for Metal Sonic, and hand motions for The Golems from Capital City.
Contributed by ShyanVixen
A splash screen for an unused VR mode exists in the files of the PC version.
Contributed by Operationgamer17
SEGA released the Sanic T-shirt as free DLC for the game. The shirt is based on the "Sanic" meme, a poorly drawn MS Paint version of Sonic the Hedgehog.
Contributed by KnowledgeBase