So I think there's some room for improving Destruction and Affliction in PVP via the PVP talent spots that will be opening up with the baselining of Curse of Weakness and Curse of Tongues. I'm not too knowledgeable about demo so I'll leave that out for now. I'm pretty happy with the already added Amplify Curse, even though I still think curse of recklessness is probably not great and its extra effect feels a little forced.
Affliction:
Imo the main issue with affliction is that simple DoT damage isn't very strong anymore. Because we have dark glare (which I don't like in general) and other multipliers like Death Bolt, a lot of our damage is focused into a burst window that is hard to realize in PVP because of how much casting is required. Furthermore many classes have at least a little selfhealing, enabling them to more or less passively outheal our weakened rot.
To address this I'd suggest adding a new passive PVP talent that stacks up a minor healing absorb whenever agony or corruption deal damage. Basically countering healing without making dot damage so high that it gets out of hand when you can freecast or against classes that have no selfhealing.
Fel Afflictions: When your corruption agony or doom deal damage, they apply Fel Afflictions, reducing all healing taken by the target by 3% of your spellpower, stacking up to 5 times).
I made it a flat amount to hit little heals like leech and hots harder, without affecting bigger casted heals too much.
Destruction
Imo Destruction in PVP suffers from a similar issue to affliction, which is a high amount of damage concentrated in in a burst window in PVE (Grimoire of Supremacy +Infernal).
1. Focused Chaos.
Instead of blocking its use on havoc this now adds a 15 second cooldown to chaosbolt, spends all shards and increases its damage by 50% plus an additional 20% for every soulshard spent. The goal here is to break the synergy with grimoire of supremacy in PVP or at least make it less potent, and make the gameplay more focused around setting up for one solid burst, without being too reliant on long cooldowns.
2. Shadowflame Then to replace Weakness/Tongues I think bringing back the old Shadowflame would be very nice:
Inflicts 150% spell damage plus an additional 240% over 6 seconds to all enemies in a cone in front of you and slows all enemies struck by 70% for its duration. 20 second CD.
3. Improved Shadowfury Lastly I think destro should have an option for a better shadowfury, since its originally a Destruction ability. I would go with a passive that decreases its cast time (and GCD) to 0.5 seconds and makes it deal a decent amount of damage.