Start out with an idea. If you don't have any ideas for a new gametype, I suggest thinking of something which could be challenging, then deciding how people could get points from it. Also, look outside of Jazz for inspiration. Other games often have exciting gamemodes, and sometimes you can steal ideas from non-videogames. For example, my entry for the Revenge of the Single Screen CTF contest. It can be downloaded alone here or with all the other entries here.