Just a small introduction. I've Been playing Demonology since cataclysm in Arena/RBG, achieved multiple gladiators/hota/r1 as it and I have seen changes to the spec for the better and the worse.
First i'd like to say, that i'm happy with demonology from a PvE perspective, it sure has been an improvement compared to legion. But, I and many others (from the pvp community) are a bit let down with its current state in pvp, mainly since we lost doom as baseline (and has been reworked) and demonwrath. These were two spells that were essential to our playstyle in pvp since the big change in legion, both of them had big impact for sustainability when we had to kite. Now demonology feels very awkward and gimmicky in situations when you are focused/targeted, eventhough with the new mechanics with Demonic core/power siphon, which are supposed to be tools to be able to deal with this. Doom has also had it's rework, and in no way does it feel impactful, even when not having to deal with no decurses, it feels very insignificant. Also we've had a talent (honor), singe magic since launch of legion, which has had no use because for it to have an effect you have to summon an imp, and losing a felguard which is much more sufficient in pvp situations because of its mobility, defensive/hp pool and utility (axe toss, mortal cleave), it has a much bigger impact than what an imp has to offer in pvp.
I'd like to propose a few ideas:
Singe Magic, make it not be required to have an imp out, but increase the cooldown of it, because otherwise it would be too powerful for PvP.
Doom dispell protection which we did have in WOD (pvp set bonus), where it granted demonic fury if it was dispelled, but in this case it refunds a soulshard or summons a demon to fight for you.
Demonwrath as a honor talent, i'm sure a lot of people would be happy if we had this ability back as an option (read at top).
Master summoner now instantly casts Demonic Tyrant and dreadstalkers.
Fel Lord currently feels very underwhelming, Many probably wouldn't mind if it was at 10-15% per hit at this point, it has big outplay potential i.e ccing it, peaking in and out to bait cleave...
Call Observer's Range and hp pool increased, is something that could be looked at.
I'm sure I have many other ideas in mind, and these were just some that came to mind now and i'm going to constantly update it. Hopefully others would like to share what they have to say about demonology in its current state or warlock class in general.
Thanks.