Near the climax of the movie, the grandmaster gives up his right to reincarnate to Kumatetsu. This right allows Kumatetsu to be reborn as a God. Later, we see that the grandmaster can no longer reincarnate into the god of decisiveness as he had finally chosen earlier.
I originally thought that all you had to do to reincarnate was become the grandmaster, but that would make becoming a god too easy. As you could keep the line moving pretty quickly if everyone became a god as soon as they became grandmaster.
So it brings up the question: once you become a grandmaster, what must you do to earn the right to reincarnate as a god?