Before osu!next will come, I might share some new elements I've found (and used in my latest skin). Take it or leave it.
welcome_text.png
(750x156 HD, 375x78 SD)
Remember the text that says welcome every time you open osu! (when "Interface Voices" is enabled in Options)? You can customize it with this element. Size isn't restricted nor tinted to a specific color. Be aware that you must have a supporter tag in order for this one to work. Works in any build.
fps-(n).png, fps-comma.png and fps-fps.png
(24x28 HD, 12x14 SD), (16x42 HD, 8x21 SD) and (54x28 HD, 27x14 SD; Image has an opacity of 50%)
These are the numbers that will greet you real-time at the bottom right of the screen (if you enabled "Show FPS counter" in Options). Size isn't restricted nor canvass is tinted but it's HIGHLY RECOMMENDED to use the standard size of these for very obvious reasons.
There should be more skinnables before we say goodbye to our current skinning methods.