I miss the old Java applets and Flash animations of the mid-to-late 2000s. They're long dead by now; Over the years it became increasingly obvious that giving servers the ability to run arbitrary un-sandboxed programs on their clients was Very Bad.
So far, I haven't seen anything similar to applets for WebAssembly. A relatively simple, standardised API for user input and graphics would be cool for making little games and tools without needing to spend time focusing on everything around the canvas.