Denathrius should lead whatever dread lords remain and whatever other vampires/gargoyles/whatever remain loyal to him.
Leading the Scourge? Leading the Legion?
No. We’ve already beaten Denathrius once. If a new leader emerges for either faction it should be a new character that is given proper development and story set up to not only work as a character leading these legendary villain factions, but can also serve as an intimidating, valid, threat to Azeroth.
Denathrius, already being defeated and humiliated once before, can never fill that role. Also, he has some charisma, but his backstory is just meh. A new Lich King needs to fill Arthas’s shoes. The only way to do that is with a new character, preferably one who goes through an entire rise and decay like Arthas did.
I loathe most of shadowlands because of how inconsistent it was and how it violated original warcraft lore. And so any “what if” I could think of would not be what Da-loser had ever intended…just something that could be done with the story hook leftover.
But if people “wanted” Denathrius back…I can definitely see where to hang something on a hook with him. For example, you could reveal that Denathrius originated among Cosmic Disorder, not Sargeras…but the Disorder that existed before Sargeras was ever tricked and turned. Then you could have him sneaking his way into the Shadowlands whenever that was created, and injecting his own magical soul into the robotic Eidolon body that was going to lead the new island of Revendreth. Then Denathrius would play the game of being an eternal one even though he is betraying them all and using them all. And since Denathrius created the nathrezim, he can be responsible for making them deceive Sargeras in exactly the way that Metzen wrote the nathrezim to have done in the Burning Crusade writing that he did. Also, since the nathrezim were part of the scarlet crusade conspiracy…then it is entirely possible for either his nathrezim to be secretly interfering in the Arathi Empire that we haven’t seen yet, or even Denathrius himself showing up in a disguise every once in a while in that empire to lead it in the direction he wants.
That prophecy written about Renilash with the arathi soldiers needing to weep at all the sins their emperor may have done…would be a perfect reveal to show that Denathrius the First bloodied lord of Sin was tricking their entire empire the whole time.