Gems are a great way to add to making some money to further develop the game,monthly subscription is also another great way,adding new skins for character avatars would be nice,maybe obtainable with gems also. Possible xp potions maybe,also with gems. There are just so many possible ways to market certain things it's hard to guess what may or may not work. You could also charge a small fee for say each extra big content adds like these games from bungie on ps4 lol