There are just so many to choose from. The first thing to pop into my head was that I would want to be a master at throwing knives. Is this a skill I can use? No, not unless the zombie apocalypse arises and even then unless there is enough oomph to behead the corpse in one throw it’s a non-starter.
Then archery took it’s place. We only did the unit twice in PE but I liked it enough (and remembered enough) to then teach/oversee the activity at summer camp when I was a counselor. Being a master of the bow could be fun. I imagine it would also be slightly more helpful then knife throwing because at least I could attempt to hunt with this skill, but I’m not sure if I have a hunter mentality, so perhaps as equally as useful for yours truly.
Diverging from the Deadly Arts because with my standard levels of gracefulness I’d more than likely use the art on myself. If I could master anything though, by magic, I’d truly want it to be spelling/grammar. Sure there are other things that I could choose that would be more fun (like juggling or the guitar). I think part of the fun of those items is overcoming the difficulties and putting in the time to become a master (the whole life is a journey, not a destination thing). There is no fun to be had with spelling and grammar. The main benefit is that I would save face a bit more often. Also that is something that I use everyday, so I would be appreciative always for such a gift. So although it’s très boring, that’s my choice.