While some people still criticise Sega's decision to effectively split Sonic 3 into two releases by holding back content for Sonic & Knuckles, I still think it's one of the coolest hardware gimmicks of all time.
Being able to 'unlock' bonus content by combining cartridges is something that appeals to me for some reason (which is why, when a new Evercade cart lands, I always spend a few minutes seeing if combinations unlock any secret games).
I'm frankly amazed, then, that an indie Sega Genesis / Mega Drive game released in 2023 also includes this functionality, yet I've only recently become aware of its existence (thanks, MatteusBeus).
The game in question is Arkagis Revolution, a top-down shooter which boasts some amazingly smooth Mode 7-style rotation effects.
When combined with Sonic & Knuckles, a new secret game is unlocked that fuses Sonic's bonus levels with Arkagis Revolution's gameplay.
If you'd like to try this yourself, you can order Arkagis Revolution from Broke Studio's Homebrew Factory.
Alternatively, if you just want to play the main game, it's included on the Mega Cat Studios Collection 2 for Evercade.