They should just make Luffy Shichibukai. After all, the Straw hats aren't real pirates as it is: They don't rob civilians or assault civilian ships or port towns. They only fight against other pirates and criminals, as well as corrupt officials. Actually they are Shichibukai already in any other manner but officially.

But considering how corrupt the world government is, I doubt they would do that. They need corrupt or otherwise twisted people in the Shichibukai, not warriors of justice.