There needs to be a shape shifting race in my opinion, but instead they shape shift into mobs not races. Races is way too op, but shape shifting into a mob can be used for sneaking into bases but needs to be limited so they can't stay in to form for more than 5 minutes. They can't do anything that mob can't do during that time. (Though they can change back to normal when they want.) And this part is debatable. But I'd say one of the three choices.

 

One change a minecraft day (20 minutes), one change a minecraft month (2 hours and 40 minutes.) Or 5 changes a normal day. Can't decide which is the most fair to use. Probably none of them but these are as close as I care to think up in 30 seconds of using my brain.