So $60 for the base game then another $20 or whatever for an expansion only a couple of months out? What's wrong with this picture?
DLC's, expansions - yeah, they're both "whatever" to me; all marketing BS. They (developers and publishers) can call them whatever they want b/c of size & length of the content (DLC's are usually not that big and meant to be more episodic in size - though Skyrim and GTA4 DLC's are more like expansions, TBH) or for marketing purposes (i.e. Witcher 3's expansions could be considered another form of DLC's, TBH; since they originally were only bought online) - but they're both falling in the still "More content to maybe buy later" arena.
Anyways, just like any game - a game is only as re-playable as you think and feel it really is.
I don't replay many games, but I've replayed the hell out of Vampire: Bloodlines.
Some points.
1. Sure, some games might have a bunch of levels w/out much change in its course to them b/c they're linear - but could be replayed w/ other gamers online and it might feel like a different experience b/c of who you played with and how you played the missions/level/area/whatever; Left 4 Dead and Vermintide here come to mind.
2. Some games, can be replayable b/c you can approach a quest, missions, area and/or level with different classes, styles, skills, weapons, etc - ARPG's like Diablo are good at this.
3. Some games, have different branching pathways that can go in different directions w/ different quest outcomes; different things happening to areas and/or NPC's; and/or actual game-endings/final permutations - regular RPG's often do this, like Planescape: Torment or The Witcher series.
Hell, one game could do a few of these or all of these things.
I think from what I played of the PC Beta, Destiny 2 just might be one of those games that probably use Points 1 & 2.