1: Felshroud or Demon Form
This is the part where I might get hate. Shadowform was originally gonna be for us, and it looks so badass on spriests that it makes me feel regretful Blizz decided not to give it to us. I promote some glyph of some sort for Destro, like Shadowform with a graphic like the glyphed shadowform but green. Stats would be a bonus to Fire damage and + haste. That would be too cool. And you don't have to agree with this, I just want to get my opinion out there.
2: Warlock Flight Form
Take the meta form and allow us to fly with it for unlimited duration. Their flight animation is amazingly cool. It could also be a glyph or spell and for demo for a glyph or all specs if a spell
There, I got that out. I can smell disagreement but whatevs