Aside from Binding Demons, we don't know what the D4 Warlock skill set will be.
I mean, we actually do, in some detail, because IGN accidentally published an extremely long video preview, and whilst they de-published it a bit later, it wasn't until after multiple streamers had done "react" streams to it (which included showing it in full), some of which are still up I believe.
Oh hey here's a full re-share of the video:
Basically Warlocks can:
A) Summon demons - seemingly more temporarily than a necromancer, some of it is more like Spiritborn "summons" which are basically just a glorified attack, but other stuff is actual summons.
B) Turn into a demon and be tough and fight in melee.
C) Put down sigils on the ground which explode and do stuff.
D) Go invisible and kill using lots of status effects.
Obviously all of this would be combined to some degree.
I theorize that one reason its taking so long is that they are waiting for BOTH the Paladin AND Warlock from the Lord of Hatred D4 expansion to release so that way they are choosable class options for this.
It's possible. I note they didn't even include Spiritborn in the original KS announcement though. So I'd be slightly surprised if they included all three expansion classes in the basic release. Maybe they'll make it part of the KS goals?
Another complicating factor of course is that this expansion also majorly reworks and adds to the abilities of existing classes.