I think it would be nice if the user could select a custom base image to experiment how this affects real-world cases.
Ideally there'd be an option to generate the mip-chain, and another to actually load each mip level explicitly.

For example, the game "The Legend of Zelda: Wind Waker" on GameCube used explicit control over mipmaps to achieve an artistic effect (fading water waves in distance).

Another example, would be maps of cities, which is a common application of WebGL. It might be interesting to see how mipmaps affect labels / text on the map (here, it might make more sense to generate the mipmap chain automatically).

I think it would be a good idea to allow negative mipmap bias. Unfortunately I had to use Google Chrome source overrides to experiment with it.

The animation can be problematic as it can block the users view. The user then has to wait until the animation is at a point where it can be disabled.

I think it would be nice if instead of pausing, there was a slider to control the progress into the animation + a separate slider to control speed (which could be used for pausing, but potentially also to reverse the animation).