To legalise marijuana a government would have to have an extremely good reason, and there already is an extremely good reason for it to be outlawed.
Sure, some would say, "Legalise it, people are taking it anyway." But that's like saying, "Legalise murder, people are doing it anyway."
Even then, no one would really be in the right state of mind to think straight about spending tax money.
Despite the unlikeliness, I would say cancer research and environment friendly things.