One tool I'd really like to see in this format is a simple "turn the background of this PNG to transparent". Models still refuse to follow the instruction to create transparent backgrounds for logos they create, and I often have to look for other tools doing this as post-processing.
It's possible that this is too complicated for the "few hundred lines of js" code envelope, though.
Build transparent-png.html - a tool that lets you open any image and then click on colors within that image to make them transparent - showing a preview of the resulting PNG against a checkerboard pattern and optional against other selected background colors below, plus a download PNG option
It should also accept pasted images
(I’m not actually kidding)
Seriously, though, I think this solves a nicely framed simpler problem. I was thinking about a more general tool, but that's genuinely hard (you'll need heavy CV algorithms or a special ML model to detect what is background what what isn't).
To be honest, what you built here is probably sufficient anyway, because the models are better at obeying "create a white background" or "create a 0xffffff background" than "transparent", so this tool can post-process to what's needed.
When asked for "transparent", I've had a model generate a fake checkerboard pattern of gray colors to imitate how viewers render transparent areas :-) For this kind of nonsense, the transparent-png tool wouldn't do!