Desargues certainly pioneered original mathematics.  The notion of a point at infinity in projective geometry is usually attributed to him.  Kepler apparently did not work in projective geometry but rather in astronomy and pioneered a number of mathematical techniques such as infinitesimals.  I am not aware of any interactions between Desargues and Kepler, but Desargues did play an interesting role of attempting to resolve a dispute between his junior colleagues Fermat and Descartes.