My biggest one is having to cast Fire and Brimstone for every spell. I don't have enough room on my bars to make macros for every single spell affected by Fire and Brimstone nor would I want to because those macros can mess up really easily. My suggestion would be to make Fire and Brimstone a toggleable aura. You activate it and then every spell you cast is AoE but costs 1 Burning Ember (and generates Burning Embers like now). It would make Destro's AoE so much more fluid and would more than likely improve AoE dps.
My second biggest QoL issue is not being able to control Wild Imps. I know they're called 'Wild' Imps but they get rather annoying on some fights. This is mostly a soloist issue but there are times where they're attacking something they're not supposed to which causes a wipe. I would also like to have a way to control where they are, or a way to teleport them all to you because I've had numerous dungeons where patrols are pulled because of a stray Wild Imp. Giving them a pet bar along with your regular pet's bar would be great.