Rsdk — Sonic 3

Sonic 3 RSDK is an open-source reverse-engineering and reimplementation project that recreates the original Sonic the Hedgehog 3 game engine (and related titles) using modern development tools and formats. It reconstructs game logic, levels, object behaviors, graphics handling, sound playback, and scripting so the classic experience can run on contemporary platforms, enable modding, and support improvements (fixes, enhancements, and porting).

. This is a separate fan-made "remaster" that runs on its own engine but requires the original Steam Mega Drive ROM to function, offering extensive mod support and features that some feel surpass the official RSDK release. Sonic 3 Rsdk

Some advanced modding examples: