Sonic Worlds Next is the official continuation of the old Clickteam Fusion-based Sonic Worlds Delta fangame development kit, but instead made for the Godot engine. Sharb is the lead developer of this project (I'm the project manager) along with DimensionWarped (one of the original Sonic Worlds developers!)
The goal is to provide a complete basis for creating classic Sonic the Hedgehog-style gameplay that is as accurate to the original Genesis/Mega Drive games as possible. So far, I say that we have nailed it! Sonic, Tails, Knuckles, and even Amy (in her Sonic Origins Plus incarnation) are fully implemented and working as expected! There are a good deal of pre-made gimmicks available for use and as reference for creating your own stage elements, and we have frameworks for boss fights (a primitive sample boss is provided) and handling special stages (a simple win/lose debug scene is provided).
Sonic Worlds Delta was provided under an informal "if used, please give credit" license, whereas Sonic Worlds Next is provided under a more formal MIT license... which basically equates to the same thing.
There are some games already being developed with this! I'll let Sharb post her project.