While it seems like this is just a shift from using glyphs to having warlock pet customization use Grimoires, and a questline for... *checks notes* A new kind of Imp... for some reason...
I am going to renew my call for Blizz to add options for warlock minions to pull from ALL kinds of forbidden magic, not just Burning Legion Demons.
Warlocks should have undead summon options of all sorts. (Scourge sets, and ones from SL, yeah SL was a mess but hey might as well get mileage out of their models)
Trolls (Darkspear/Zandalari) should be able to summon weird Ghuun blood monsters and undead allowed by Bwomsamdi or even the captive Mueh'zala.
Humans/Kul'tirans should be able to summon Drust creatures.
Velfs, Mag'har, and such summoning Void entities. (The voidwalker is ALREADY a summon after all)