So since we have new Expansion focusing on void why not add talents like :
Shadowflame we have it but add extra option for it to deal dmg either burst or aoe dot ?
a talent that change only few fire spells into void and void looking (and make them do completly something else but still same dmg) as void.
and Add skin or talents for void summons dealing void dmg categorising them into sucking health and transfering small % of it to warlock as shield or healing over time, DPS single or AOE and tanky.
Warlocks don’t dab only in fell they do also in void too
Warlocks already use a mix of spells from both Fel (Fire) and Void (Shadow).
As you noted yourself, there are already Shadowflame spells; Demonology warlocks combine both Fel and Void energies when they cast Hand of Gul’dan or Demonbolt, clearly vislble in spell animation that combines both green and purple energies.
Destruction focuses on Fire, both regular and the Fel variety if you learn the Codex of Xerrath). Affliction on the other hand leans toward Void energies.
Now I reckon that you wish to see some Void entities and their traits like tentacles. I’m afraid that might not happen, as Warlocks tap into the magical energies directly and command Fel and Void by shaping them into spells of their own.
Command the powers of creatures native to the Light and Void realms is the domain of Shadow Priests, and I would deem it unlikely for Blizzard to make such changes, both from lore perspective, but mostly from a financial one: spending resources on visuals for spells that do not really need new ones.
So if you really want to see them tentacles and void beasts, Warlock would not be a class for you.