I thought making a siginificant change to the most influential figure of the sixteenth century would be a good way to start messing about with things.
Given Luther's long running academic career, I thought it would be somewhat interesting if he also studied magic as well as theology. I was thinking fire would be a great and dramatic path for him, but handsofblue correctly pointed out that his background seems to point more towards being a water adept.
It also allows us to assume some further interesting events. Certainly if he's a reasoanbly talented magic user then the confrontation with emperor Charles will mean there will likely be another mage present on Charles' side to counter any of his use of magic.