A new class/race could be fun, but how about more options to customize our existing ones?
We should be able to scale our characters somewhat; I can't believe 14 years after release I still can't make a fat human or a short draenei.
How about a really tall blood elf or a skinny pandaren? Why not?
It doesn't have to be a massive difference, maybe like 25% bigger or smaller from standard.
How about being able to choose how big my weapon is, or where I sheathe it and at what angle?
A lot of this stuff should be pretty trivial to implement, which makes me sad it hasn't and probably won't ever.