When using my 32 inch screen sometimes I set the wallpaper to a random walk video in YouTube and it works fantastically. I know that it is counter intuitive, but the contrast between real and moving elements and static and almost flat Windows makes for a nice experience. And helps fighting text editing fatigue.

Of course, ymmv. But give it a try.

That's up to personal preference. Just like with static wallpapers, some people like single subtle colors, some people like noisy multicolored wallpapers.

There's already subtle parallax in iOS etc.

Animated wallpapers could be subtle, like raindrops, or animate only when interacted with, when you're bored or waiting for the AI to finish a run etc. :)