My top 5 in order.
1 2nd account - You double your customers value per sub. This is a huge quality of life that will directly transfer over into player retention as when you get board you can use the 2nd character to try something different.
2 dimension runs - This game needs on demand group content. Most of the content in this game is solo even tho you need protection from bigger groups. This will give a great group gameplay loop giving value to the players time while rewarding small group gameplay.
3 Combat and magic rework. There are some very OP spells but the schools are not fleshed out. We need to fix ritualism, make spiritism fun, nerf a lot of the elemental damages, nerf ecumenical a little but quicken the cast speed a little along with some defensive spells.
Nerf towershields reduce, round shields weight. Rebalance 1h so they are not so strong. Nerf ogh beer belly and piping. Reduce 360 movements. Add a penality like statloss from MO1 to fix the naked griefers for those that spawn outside of town, and making pet leveling have more quality of life.
4. Bug fixes - People are paying for a sub and the game has a lot of bugs and unpolish. You need to start fixing stuff after to give a little bit of content to tide the players over.
5. Brokers - the quality of life of not having to wait on your tamer friend to log on to manually trade you horses will save a lot of people time and annoyance