I had an idea once but not the time nor motivation to build it: a database of game design with links to gameplay snippets.

So for example an article on "2D platforming" that discusses the implementation in Super Mario, and includes a "demo" button which launches a web emulator with a save state that demonstrates a specific jumping section of the game.

Legally perilous maybe, although my non-lawyer brain sees that as fair use, especially if the emulator doesn't let you play the full game. Idk, but it'd be a unique thing on the internet.

Edit: this is awesome btw, im definitely setting up a personal instance soon