Let's brainstorm a bit!
As you know Affliction in BFA (except for the very short time when the meme drain was a thing) has been a complete trainwreck of a spec in just about any form of pvp, but most importantly 3v3 of course.
Why? Primarily because of melee cleaves being able to utterly shut Affliction down and deal frankly unhealable damage to it.
The spec soaks up damage like a sponge but doesn't deal any in return.
With one spell school only, no baseline Demonic Circle, no way to peel for ourselves and no other mobility to speak of, other than a lenghty cooldown, gateway, we are in a terrible spot. In addition to that in BFA Affliction was heavily designed to be more of a single target spec, making our spread pressure, especially when not being able to burn cooldowns and cast UA, due to weak dots, well lacking.
What changes with Shadowlands? Demonic Circle is baseline once again, allowing us to take either Coil or the new Talent 'Tongue Tied' which works as a heal absorb. Instead of stacking UA we'll have a new soul shard spender 'Malefic Rapture' which makes all our other dots deal damage. Former pvp only Curses (Tongues/Weakness) are now baseline again, which leaves space for several new pvp talents and this is where I'd like to voice a few ideas of mine and I'm eager to hear yours.
In my opinion, Affliction badly needs some way to get melee off itself and this is why I believe Shadowflame should return, in a sligthly reworked fashion. Namely it should root instead of slow. This would create much needed breathing space to actually get off those Malefic Raptures and other casts.
The next new talent that should be added is one that formerly existed aswell, altough the name currently eludes me. Basically Fear has become a worthless spell to use on any target that is already dotted up, as it breaks almost immediately. This is counterintuitive to the gameplay of the Warlock and ideally wouldn't be a problem. Assuming fear breaks just as easily in Shadowlands as it does in BFA we need a talent that causes Fear to momentarily stop the dots from dealing damage, but preserve them and once the Fear has worn off, they continue dealing their damage.
Finally I'm hoping for either a new baseline ability (Covenants aside) that we can use when being locked out of Shadow. It feels absolutely terrible when you get interrupted as an Affliction Warlock as there is literally nothing that you can do. For the love of god, we need back either Searing Pain or Immolate, or both. Affliction used to be THE Warlock pvp spec and I know many of us still love it dearly and wish to merrily spread the rot again in Arenas and bgs. We need some BIG love.
P.S Demon Armor should be baseline