If you just ran out of things to do in Destiny 2 vanilla, your first DLC expansion, Curse of Osiris, adds some new activities for you to carry out. For the most part, however, there is not enough for the expansion yet to justify the return. My opinion is still in constant change since I have not played the Raid Lair yet, but until now, history missions, Strikes, new Crucible maps and Adventures feel more of the same, despite the new configurations and stories of the DLC.  Osiris' curse begins right after the end of the base game campaign, as far as your level is concerned. You could go directly from the end of the history of the Red War to the Curse of Osiris campaign, which requires a power level of 200 to 220, without having to grind a lot in the middle. For newcomers or PC players who have had less time with the game, it is a comfortable bridge to level up between the lower-level cutting-edge content and the high-end activities of the end of the game such as Nightfall. (However, those final activities are a different story, we'll get to that in a moment)
As a result, however, the Curse of Osiris history missions feel like filler. The campaign establishes a huge company against the Vex, with infinite timelines and computer simulations and the mysterious sorcerer Osiris mixed in everything. But with a run time of two hours or more, missions rush through interesting concepts and lead you to a simple final battle that is essentially scripted. Nor is it enough time to fully understand Osiris as a character, which is disappointing considering that it has only been mentioned previously in the world of Destiny. The infinitely designed and varied Infinite Forest, a Vex creation designed to simulate timelines and their endless permutations, is the most interesting addition to the expansion, but even so, history does not force you to explore it, but guides you through of areas to find the codes and things that the smartest NPCs can use to identify their next destination for you.
In addition to the Infinite Forest, the new destination, Mercury, is simply uninteresting to explore. It is a small circular map with a new public event to try, a new supplier and a handful of lost chests and sectors. The basis of exploration established in the base game is still good here: having a variety of options to choose from makes things seem less repetitive, but it feels like heavy work with little to do at the highest level. That extends to the new strikes, which are almost direct copies of two of the missions in history, nothing more than another way to kill time.
The biggest problem with Curse of Osiris is that it blocks the most difficult activities, including the Prestige Nightfall and the Prestige Raid, behind its new power level limit of 335. The recommended power for those activities is 330, which does not You can reach if you do not have the Curse of Osiris DLC. So, if you do not get the DLC, suddenly you do not have access to something you used to be able to do. It is also frustrating if you get Osiris Curse, because the higher level requirement does not fundamentally change these activities.
The New Heroic Adventures add Nightfall-style modifiers to the Mercury Adventures, but those missions are not begging to be played. The main incentive to do so is to unlock the NPC Brother Brotherhood's Lost Prophecy mission, which finally unlocks the Forge. From there, you can make legendary Vex weapons. But for anyone besides the most dedicated players, there is no compelling reason to do all this unless you want to redo old missions in harder difficulties to get the loot to use when you do it again.
Excellent shooting is not enough to sustain an expansion that adds little out of busywork.
While some of the new booties are worth picking up – my favorite of those I've found is a legendary auto scanner rifle – you're likely to get a lot of duplicates before you get something you really want to use because the main one Reward for everything you do is a new and bright booty, the rate of frustratingly high duplicate drops makes grinding more disappointing than satisfying. However, shootings feel as good as ever, so it can be fun to experiment with new weapons, but it is not enough to maintain an expansion that adds little extra ways to occupy your time.
That said, you'll still need more time to try out Lost Prophecies and the Forge, as well as Raid Lair when it's released. If they provide more than one game ending and have a broader purpose than filling in the same kind of things, I will be more inclined to return to Destiny 2 than I am currently.