I've been a part of a couple beta cycles now, and I know there's some things I wish I had access to that aren't available as part of the regular beta experience. These things would allow for more focused testing.
For starters:
I wish we had some way to access copies of all adorns in the game as well as a way to select for the quality of an adorn. The assortment of adorns we start the beta with aren't sufficient for testing purposes. Actually, please just extend this to all the gear in the game. If I want to spawn 3 copies of Old Nothren Crowns so I can stuff them with different adorns to test different playstyles I'd appreciate the ability to do so.
Also, can we get a way to buy stacks of each material in the game? This way we could test out different ascension levels. Alternatively you could just crank up the multiplier on the refinery to like 5000% yield and we could get our mats that way.
For pets, please give us access to the dragons from the Draconic Era event.