1. Experience from quests. My friends quitted because after doing all available main quests they just got level 50+ and almost 1-3 levels per week after that. So they got bored and did not play anymore because mostly they dont even see people doing quests. Maybe add a waiting channel, for people that will do the quest with people so they will not be alone doing it. Maybe increase the experience of main quests so after completing it they can run graveyard.
2. Luck. Bring back the luck farmers by adding more items with luck. Maybe add a weapon, helm, armor, belt. Gold loot items are very op now and everyone is almost relying on gold loot to farm. Bring back hunting for vials, locks, gears. So item prices will be balanced.
3. Vanities. Make the old legendary,mythic and arcane weapons or equipments to vanities. Make it craftable. Glint set is a good vanity with glowing effect also the banished set. Glint weapons are also good as vanities. Make them like the cryo weapons. So people can still use those items. Or maybe make it craftable to increase the levels of the items and adjust the stats.
4. Additional Classes. Maybe counter part of rogue - assasin, mostly short range skill attacks unlike the rogue with long range deadly attacks, warrior - paladin, mostly taunt crowd control or increase in stats or heal to sustain in boss maps, sorcerer - wizard, more aoe skills to kill many mobs. Or a new support type of class like priest that supports the party in boss hunts. (Name of Classes is from Ragnarok Online)
5. Gender. It will be awesome if you make a warrior that is a girl and a rogue that is a boy. Because we have marriage system. Maybe it is more exciting to express your real gender. And maybe add a church map for people that is marrying. So other people can watch.
6. Market Place. Maybe make the marketplace the starting map. So new players can see old players and ask for help. Or maybe add a chat box for selling and buying stuffs for convenience.