Hey, warm greetings to the reader.
This post and the suggestions therein might be absolutely out of scope to implement in a x.1/2/3 patch, however for a new expansion with a bigger class and combat revamp and more development time it might be worth it to offer this feedback/suggestion.
**Mobility: Instant cast blink with a twist to differentiate between Mage's Blink**
Think a very good addition would be to have a somewhat longer range instant cast teleport with a longer cooldown. To differentiate it between Mage’s Blink you could give it no charges, like one port every 30, 45 or even 60 seconds (think Venthyr covenant ability but without cast time, instantaneous, be it with a reticle or without) + cool darker animations and visual effects attached to it.
Demonic Teleport with Circle is very deterministic and Gateway can be a bit of a hassle to cast, even with Soul-burn attached to it, making it instant cast. In the heat of action (be it a mythic raid encounter or a high key with a boss fight that has a big swirlie cast on you) even that 1-2 second can be too much of a hassle with having to manually mouse over place the gateway at the target.
Having played a mage character for a bit in Season 1 during War Within really made me realize how very practical and versatile a blink / instant-cast teleport can be in all manner of content. Be it Zek’vir ‘??’ or any slice-mechanic in the raid (like Nexus Princess during intermission). Burning Rush with it’s global cooldown and very punishing life drain is a bit rough in higher end content.
Giving the Warlock an equivalent (with the above mentioned differentiations for example) in addition to the pre-existent Gateway (that you can use only once every 2 or 3 minutes? not entirely sure about the time-gate) and Circle (which again, is very deterministic and requires a pre-planned placement, not applicable when suddenly targeted with something and the need to get out really fast + only usable once every 30 seconds) would be amazing!
**Unstable Affliction as a hard hitting single target DoT similar like in Legion**
Also advocating to make Unstable Affliction a very hard hitting dot that can be applied ONLY to a single target. In a span of 6 seconds, it would deal the damage-equivalent of like 5 fully empowered Malefic Raptures. Would have to be definitely tuned appropriately to not blow out every other spec and class in the game out of the water in a single target scenario but think that this change coupled with the the mobility changes advice above would bring Affliction Warlock on the top of the charts once again (see Legion raid statistics on warcraftlogs com for reference). A quick idea (brainstorming-like) would be to allow casting unstable affliction only once certain conditions have been met (like having at least 3 DoTs up on the target).
The above written suggestion about the hard hitting Unstable Affliction (6 second single target dot which would deal the equivalent of 5 fully empowered Malefic Raptures) would also give the Affliction Warlock a very powerful niche in higher Mythic+ keys where priority mobs and bosses need to die asap. Mixed with 2 other dps that bring great overall (AoE) damage it could make the Affliction Warlock shine in certain compositions in the Mythic+ environment as well when played efficiently.
All the best
God Bless