I have decided what we should do with the angry cat (T193330). Replace it, if you want.

Each project will now be able to define its own background image by setting $wgTimelessBackdropImage. (We can just add an array for this to the wmf-config InitialiseSettings.php a la the mf logo config.) The question, of course, is what do folks want to replace it with?

I made a couple example background images just based on the project logos: for wiktionary and for meta.

But honestly, there are so many options...