Introduce a simple old school RPG-vibe adventurer's log 2D portrait/avatar for players' character. A few (20 or so) could be premade ones from the Developer team (the picture of a generic warrior, wizard, archer, ect.), while if a player wants to get a custom portrait/avatar or upload one him or herself (the picture/image must follow the medieval Old School vibe/theme of the game), they must pay in-game gold per week/month to maintain it. Each time a player wants to change their custom avatar/profile, they must pay in-game gold too (at the same price as getting a new custom character portrait/avatar).
This is a good and fast method of gold sink, is easy to introduce in-game, and does not detract from the game's core mechanic/gameplay