I would like to see a way to personally test class/spec, particularly for trying to decide what class to play going into a new season.
A great way to do this would be through the Proving Grounds, or something like it. Add a vendor in there that is kind of like a tournament vendor who gives you any gear/gems/chants you want while within that instance. And then have events like the Proving Grounds that you can participate in to test, as well as training dummies.
You can only test this within the instanced Proving Grounds so it will have no [direct] affect on other players.
It would be a great way to better help decide what character to play–again, particularly going into a new season.