DXT/S3TC texture support
Compressed textures can be very beneficial for both performance and VRAM usage. DXT1 and DXT5 (S3TC in OpenGL's terminology) are by far the most common compressed image formats on desktops. Adding internal support for using DDS (DXT) images would be a useful feature, and could pave the way for easier addition of the ETC and PVRTC compressed formats very commonly used on mobile devices with OpenGL ES.
Currently, DDS files can be loaded by LÖVE, but DevIL will convert the image to standard uncompressed RGBA before OpenGL can touch it.
Adding clean and intuitive support for compressed textures might require some reworking of the Image module internals, as well as consideration of API issues relating to the Lua-side ImageData methods for reading from and writing to the image's pixels.
Here's an overview of DXT and similar compressed texture formats: http://renderingpipeline.com/2012/07/texture-compression/