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…
"Playable Quotes for Game Boy Games" - https://www.youtube.com/watch?v=z9JYOZWLMlo