The game that comes to mind fastest for me is the MUGEN project, Card Sagas Wars. MUGEN is basically an open fighting game engine, in which people construct their own fighting games, often with existing fighting game characters. It’s where videos of fights between Peter Griffin versus Homer Simpson are made. Typically, people create MUGEN builds that are just massive super-rosters of every fighting game character from every fighting game, and pit Sub-Zero against Dark Ryu and shit like that.
Card Sagas Wars was one of a few MUGEN builds that was going to have entirely new character sprites and movesets unique to them; basically, a new fighting game. The roster was a who’s-who of copyright infringement: Chun Li, Donkey Kong, Cloud, Samus, etc. The sprite work was incredibly impressive, each character uniquely rendered in a 16-bit chibi style, and the combat looked fast and deep without overreaching into the unblockable chaos that MUGEN builds typically favor. Here’s a video:
It had been in development from what seems like 2007/2008, but around the early 2010s, the devs stopped posting updates. I looked around recently, and read somewhere that a fan had contacted one of the few people responsible for the build and found out that he had started working in the games industry proper, hence the project’s cancellation. So, good for him!
I’m not sure I would want this game any longer, but it sure as hell was a compelling and impressive fan project.