I'm wanting this feature too!

Even if it wasn't truly transparent, being able to select a background color aside from white would be helpful.

It would be useful if you could attach a download in a different file format.

I've seen a few videos of 3D models. Artists could offer the model download with this feature.

I've also submitted several vector images which have to be converted to png for the website to support them. It would be useful if I could include the svg vector file on the download button too.