Realistic rendering of comets
At the moment, Stellarium supports comets as solar system bodies, but displays them as star-like objects. More realistic and more scientifically accurate rendering of a comet requires the rendering of four elements - a core, a coma and two tails, oriented according to the comet's relative position to the Sun and its direction. The tails and the coma should appear only when the comet is close enough to the Sun. (See the Wikipedia article on comets for more information.)
As not all comets are identical, this feature should allow comet customization. The visual characteristics of comets should be stored along with their orbital elements.
Requirements: OpenGL, some mathematics and astronomy.