My idea is to have a voronoi texture be drawn over slowly-panning concept art relevant to the stage. For example- this could show in a late-multicellular setting.
It probably also would be a dark gray gradient instead of black. Above, there should be a title saying the save game (with "new game" displaying if there is no save name), and a sub-title with the name of the stage. Below, we have a witty string stating what the computer is doing, unlike Spore's "reticulating splines" which don't correlate to a process. Percentages could be difficult to show, as there would have to be a cache of all species done to see what has to be done. (But only the region will need a loading screen to calculate CPA, other things can be done while the game is being played.)