All what we have is the old plain boring black Background i think everyone would aggree that they can add an image (not saved by saves because of filesize) and having not always to look on that dull black. I think GIFs (animation support) would be awesome, because also you would have nice Sky for it and we can see what the community has to offer
There was actually a glitch I found on some machines where at certain pressures, the background would become transparent and show behind the TPT window below the particles. I'm sure that could be done deliberately, if it happens by accident.