Let's take a look at how the DLC narratives in D1 vs D2 have played out. In particular, I want to address the ideas of a character's narrative integrity, and giving a satisfying ending to a DLC WITHOUT killing the main villain.
In the vanilla campaign for D1, we had set up for The Dark Below, House of Wolves, and The Taken King via 1-off missions or the visit to the Reef for those few cutscenes.
In The Dark Below, Eris shows up and warns us about Crota, who we know (from grimoire and the Crota's Sword mission) is a pretty substantial threat on the Moon. We spend the campaign taking out his followers, listening to Eris talk about her past with him, before finally interrupting the resurrection ritual that would see him return. We save the day, there is no longer the imminent threat of a Hive God coming to our moon, the end. Anyone who is only here to play the campaign is satisfied and puts the game away, but people invested know we didn't kill him, only stopped him from returning. We now have the raid, where we launch a preemptive attack on his turf to put a stop to him once and for all.
For House of Wolves, we chase down Skolas. We spend the campaign 1 step behind him as he consolidates power and attempts to unite the Fallen, which culminates in the fight against him on Venus where we re-capture the criminal. His plans are twarted, the galaxy is safe, the end. He returns in the end game in the Prison of Elders where we finally put him out of his misery for good.
In The Taken King we meet Oryx, who has been alluded to since vanilla D1, and spend the campaign finding a way to kill him before he can kill us. We repeatedly meet with his echoes and henchmen as we do this, and his presence is felt during the entire campaign as he tries to squash us. Eventually we make it to the climactic fight, confront him 1 on 1, and "kill" him. Oryx is gone, the Dreadnaught is neutralized, all is well. Then we use the raid to bust into his Ascendant Realm and kill him for real.
In Rise of Iron, the campaign takes us all across the Plaguelands to put a stop to SIVA and the Fallen that are abusing it. This culminates in us destroying the replication chamber, preventing the spread of the rampaging nanites. The raid has us delve into the Perfection Complex to kill Aksis, who was heading up the Splicers and finally lay this threat to rest.
I want to note the idea that a character's in-game presence needs to match to their grimoire and narrative presence. What I mean by this is, if we have a villain who is hyped up to have slain 1000 Guardians unopposed (Crota), then our in-game interaction with them needs to reflect that. If we spent Dark Below hearing about the great tragedy of the moon, then Crota was revived and we shot him in the head enough times, there is nothing satisfying about that. But in a raid, where the mechanics are punishing and you're personally invested in your performance, you have bosses that feel truly satisfying to beat. So killing Crota in a raid provides an appropriate feeling of accomplishment, while the atmosphere of the raid provides the same pressure that one would expect to feel when facing such a foe.
This is why strike bosses and story mission bosses/mini-bosses work. Strike bosses are big and bad, sure, but there is little to no foreshadowing/hype surrounding them before you encounter them. They're just regular enemies that are strong enough to make a strike-level ruckus, which you're ok killing off. The same goes for those random named yellow bars in story missions.
And this brings us to D2.
In vanilla D2, we have Ghaul completely decimate the Tower. Everything we hold dear is gone in one swift, precise military strike. He managed to do what not even Oryx could do! And what kind of closure do we get? We shoot him in the head a bunch, then he dies. This isn't even taking into account how absent his overall presence is during the campaign. The game makes a big deal about him, we shoot him, then everything is hunky-dory. And rather than giving us the closure that the raids against Oryx and Crota gave us, the vanilla raid instead introduces Calus and a whole new plotline. He was only previously mentioned in pre-release material and 1 adventure in the game. Contrast that to Crota and Oryx, who were mentioned in 1-2 adventures/missions in vanilla D1, then got entire expansions based around them.
Curse of Osiris - this is even worse. Panoptes was the very definition of a throw-away villain. We don't even see him until almost halfway into the campaign, then for the final fight, we throw some balls at him until he dies. That's it. For the "raid" boss, we got an unrelated Vex mind, and while I don't mind Argos (the fight is really cool), and I find him sufficiently mysterious enough to hold his raid boss mantle, it really undermined the gravity of the CoO campaign.
Which brings us to Warmind. We are featuring Nokris, Xol, and Rasputin. Nokris was intentionally foreshadowed as a mysterious entity, given a place in Oryx's Pantheon but nowhere to be found in any of the Hive's records, and had Eris searching high and low for him. Xol is one of the gods of the Hive, 5 creatures directly connected to the Darkness that set the entire Hive race into motion. We've all read the Books of Sorrow, we know how powerful these things are supposed to be. And Rasputin, who really needs no introduction.
What we got in this campaign was a repeat of both D2's campaigns. Xol wasn't even properly introduced, had 1 bout of dialogue at us, then inexplicably charged at Rasputin. Nokris popped out of a portal then subsequently died with 0 fanfare. Rasputin basically wasn't in this DLC aside from what I can only call shoehorning in. And our raid lair has been datamined to be some Red Legion goober for a final boss.
The gameplay needs to fit the narrative. In all three of the Destiny 2 campaigns we have killed the main "villains" after having little to no actual interaction with them, giving them no development, and only doing so largely because we're told they're a threat, not shown. And we are given no gameplay closure, because the challenging activities are all relegated to a completely unrelated narrative.
But there's one last thing I want to point out: In Destiny 1, the DLCs are about the villains, while in D2, the stories seem to be about the NPCs. Vanilla D2 isn't about killing Ghaul, it's about gathering the Vanguard. We travel to each planet to rescue/recruit them, finding them in some uncharacteristic situation before "helping"/convincing them to get back in the saddle (albeit in very abrupt and jarring ways). The CoO campaign isn't about stopping Panoptes, it's about finding Osiris and reconciling with him. The Warmind campaign isn't about killing/stopping Xol, it's about re-introducing Rasputin into the plot. All of the villains in D2 are just shoddy tack-ons meant to give you something to shoot and kill to symbolize the end of the campaign.
If Bungie is going to continue to make this type of campaign, they need to embrace that narrative style. If vanilla D2 was supposed to be about knocking Guardians off their golden throne and taking a look at what it means to be a Guardian, then have our victory be pushing the Red Legion back and reclaiming the Tower, NOT just killing Ghaul. Imagine how much more satisfying the campaign would have been if we didn't have Light for all of it, and instead had to find it in ourselves to fight as Guardians even without what "made" us Guardians? That our actions and choices inspired the Vanguard to do the same, and we pushed the Red Legion out of the City even without Light, proving we didn't need it to be protectors? Ghaul fled, we got the Tower back, and we woke the Traveler up and regained our powers. VERY satisfying ending, and Ghaul is still around to be a raid boss.
Imagine if Curse of Osiris focused entirely on Osiris' past and relationship to Ikora, and instead of Panoptes even existing, we simply looked at the Infinite Forest as an entity? A giant Vex machine that Osiris was exploring, and we ultimately destroyed a central data hub or something that prevented the Vex from using it further, and allowed Osiris control over all the simulations. That would be just as satisfying of an ending; we still win, destroying something important to our enemies, without some throw-away villain that doesn't give us closure to kill.
For Warmind, imagine if Rasputin wasn't even in the DLC. It was instead about Ana trying to look into her past and all the weapons/stuff we find are just Clovis Bray research projects. Rasputin honestly didn't add anything to the campaign, and the focus could have moved away from him and gone more towards "reclaiming part of the Golden Age, and using that to drive off our enemies." Or, imagine if the campaign had been about how Rasputin froze himself and Xol to stop the Worm God, and we spend the campaign trying to stop the Hive from freeing Xol from the ice. Or perhaps Xol broke out and is after Rasputin in retaliation, and the final "fight" is us repelling Xol's attack on Rasputin's core. If we used an overcharged Valkyrie to drive the Worm away, and Rasputin was able to set up defenses to ensure he wasn't vulnerable to attack anymore, then that is plenty satisfying WITHOUT killing off Xol in the campaign.
This leads us to a Raid/Raid Lair where we hunt the worm down, knowing we can't let it roam freely, and giving us a proper raid fight to kill him.
Every DLC doesn't need a "big bad" that we have to kill to stop whatever threat. Every DLC doesn't need to be focused on stopping some big threat. But the recent DLCs definitely show a lack of solid narrative direction AND a lack of understanding that a character/villain encounter needs to have a proper narrative importance : gameplay ratio.
Going forward, I really hope Bungie reconsiders this and how they're working the Raid/Raid Lairs.