For FFXIV.

Reasons for the game feeling dead or empty? I have a few thoughts on it.

1. Over reliance on small scale instanced content. There's very little to regularly engage players in the actual open world areas. As such they tend to be empty outside of the week or so of a patch cycle.
2. Overwhelming majority of content being extremely easy and simple. Essentially removes alot of opportunity to have people actually interact with each other resulting in many players just viewing other players in duties as NPCs. FFXI was far more social. It also had far more mechanical detail than this game.
3. Too much "fast travel" Open world games in general have been doing this lately calling it "quality of life" However it's also immersion breaking and anti-social in an MMO as it removes oppurtunities to meet new people since they're all just poofing, fizzling, bamfing, flushing, etc around the world instead of travelling it. Open world games in general also falling into a trend of instantly marking quest objective locations on your map for you the moment you flag a quest is also a contributor to this. No need to ask other players questions if the game is spoon feeding all the answers to you eliminating the need to even read or listen to the quest dialogue.
4. Structured as a single player game. The game is driven on the story line in order to access content. The main story line essentially plays out as if every other player out there doesn't exist making the majority of the game feel like a single player game that has sections where you can add extra players to make it go faster. It's not to say having single player based story campaign is bad. However forcing players to go through it kind of feeds that mind set of "I'm the hero and everyone else is just an NPC" Structuring the game more as a sandbox style then filling the world with optional stories, secrets, lore quests, world or community events in game (can be simple things even like FFXI's besieged and campaign battles). Could even be something like outbreaks of a specific monster type where it keeps track of total killed by the entire player base along with personal counts to unlock reward tiers when the event ends. You can still have an MSQ in this case but not gate things behind it instead making the MSQ an option (albiet easier) to unlock specific things by doing it but have other methods available if people really do not want to do the MSQ. The best example that immediately comes to mind is also from FFXI in the form of the Airship Pass which players got for free doing their nation's rank quests or could just be purchased from an NPC for 500,000 Gil if you refused to do the quests to get it for free.