What if there were a "themes" menu? If someone wanted to re-skin SuperTux, they could do so and use it in-game. It would work a bit like Minecraft's resource packs screen, the default files would be at the bottom and you could stack more -- possibly incomplete -- themes on top, using the default as a fallback for missing files. You could download themes from the Add-ons manager just like you would for a level pack.
For example, say somebody makes a theme that switches the roles of Tux and Penny, but there are no files other than for their sprites and "speech" sounds used in the story mode, and someone else makes a theme that makes the creatures more Mario-like while retaining the aesthetic of SuperTux. You could put the Mario theme on, then the Tux-Penny-Swap theme on top of that, using both themes at once.
Or maybe it could be easier to test out new sprites as Alzter makes them?
To be honest, though, the entire reason I bring this up is because... I made a theme Right now I just have a batch script that switches them.