If you simply ran out of things to do in vanilla Destiny 2, its first DLC expansion, Curse of Osiris, adds a few new activities for you to take on. For the most part, though, there isn't enough to the expansion yet to justify coming back. My opinion is still in flux since I haven't played the Raid Lair yet, but so far, the story missions, Strikes, new Crucible maps, and Adventures feel like more of the same, despite the DLC's new settings and stories.
Curse of Osiris picks up right after the end of the base game's campaign, as far as your level goes. You could go directly from the end of the Red War story to Curse of Osiris' campaign, which requires a power level of 200 to 220, without having to grind much in between. For newcomers or PC players who've had less time with the game, it's a comfortable bridge for leveling up between the lower-level vanilla content and the high-level endgame activities like the Nightfall. (Those endgame activities are a different story, though. We'll get to that in a bit.)
As a result, though, Curse of Osiris' story missions feel like filler. The campaign sets up an enormous undertaking against the Vex, with infinite timelines and computer simulations and the mysterious Warlock Osiris mixed up in it all. But with a two-or-so-hour runtime, the missions rush through the interesting concepts and usher you into a simple final battle that is essentially scripted. It's also not enough time to fully understand Osiris as a character, which is disappointing considering he's only ever been mentioned in Destiny lore before now. The beautifully designed and varied Infinite Forest, a Vex creation designed to simulate timelines and their infinite permutations, is the most interesting addition in the expansion--but even then, the story doesn't task you with exploring it, instead shepherding you through areas to find codes and things that smarter NPCs can use to pinpoint your next destination for you.
Other than the Infinite Forest, the new destination, Mercury, is simply uninteresting to explore. It's a small circular map with one new Public Event to try out, a new vendor, and a handful of chests and Lost Sectors. The foundation of exploration established in the base game is still good here--having a variety of options to choose from does make things feel less repetitive--but it feels like busywork with little to do at the highest level. That extends to the new Strikes, which are almost direct copies of two of the story missions, nothing more than another way to kill time.
The biggest problem with Curse of Osiris is that it locks the hardest activities, including the Prestige Nightfall and the Prestige Raid, behind its new power level cap of 335. The recommended power for those activities is 330, which you can't reach if you don't have the Curse of Osiris DLC. So if you don't get the DLC, you suddenly don't have access to something you used to be able to do. It's also frustrating if you do get Curse of Osiris, because the higher level requirement doesn't fundamentally change these activities.
New Heroic Adventures add Nightfall-style modifiers to the Adventures on Mercury, but those missions aren't begging to be replayed. The main incentive to do it at all is to unlock the Lost Prophecy quest from the NPC Brother Vance, which eventually unlocks the Forge. From there, you can craft Legendary Vex weapons. But for anyone besides the most dedicated players, there's no compelling reason to do all this unless you want to redo old missions on harder difficulties in order to get loot to use when you do them again.
Excellent gunplay isn't enough to sustain an expansion that adds little outside of busywork.
While some of the new loot is worth collecting--my favorite of the ones I've found is a Legendary automatic scout rifle--you'll likely get a lot of duplicates before you get anything you actually want to use. Because the main reward for everything you do is shiny new loot, the frustratingly high drop rate of duplicates makes grinding more disappointing than satisfying. The gunplay feels as great as ever, though, so it can be fun to experiment with new weapons, but it's not enough to sustain an expansion that adds little outside of extra ways to occupy your time.
That being said, I still need more time to try out Lost Prophecies and the Forge as well as the Raid Lair when it launches. If they provide more of an endgame and have more of a purpose than just padding out the same kind of stuff, I'll be more inclined to return to Destiny 2 than I am currently.