Items that are limited in number or very difficult to obtain. Rarity will not only drive the economy but will give a reason to farm for long periods of time. People that play this type of game love to grind and many are willing to put the hours in. For that reason, rare and ‘chase’ items should be obtainable from solo content too, provided they’re incredibly difficult to get.
Diverse character setups.
This one should go without saying. Every class should have multiple playstyles and setups. No Dwarf wants to be a carbon copy of every other Dwarf in Marienburg. That’s boring. Let us choose between the skills we use, our armour sets & weapon types. Allow player skill and player choices to dictate who wins in PvP, or is better at PvE.
An in-depth guild system.
In some games, guilds function only as an extended friends list (CH.) It’s the boring execution of a potentially interesting mechanic. Guilds could have funds to manage, upgrades to purchase, payouts for clan members, a guild bank and an alliance system to name a few. Some games allow the guild leader to fund guild events/upgrades by taxing their members when killing mobs, selling items, etc. Allowing a group to contribute towards a common goal will build friendships.
A game that provides its players with the tools to build their own communities will see a longer lifespan and more hours invested.