Rise of Iron’s post-story solo content was structured around repeatable weekly quests with rotating unlocks that gave you a reason to come back each week. The Dreadnaught was full of hidden chambers and chests that required puzzle solving to unlock, and there were alternative mission paths and hidden quests that unlocked unique gear.
Maybe your best clue to what the world of Destiny 2 will feel like is to look at how they experimented with progression and side content in the previous D1 expansions. The Taken King and Rise of Iron made an effort to involve more direct story and dialogue in missions, and there’s a clear lineage in Homecoming from what they were doing earlier (with the notable addition of characters actually showing up in the mission instead of just talking in your ear). For PvE, we don’t see much more than we’ve seen from previous Destiny content. Certainly if you’re a PvP player then finding out how 4v4 matches work or what the new weapon structure feels like matters alot. The Destiny 2 beta answers some mechanical questions.