Higher tier membership, exp boosts, fixing the player model to expand cosmetics.
Before downvoting me for mentioning "exp boost", hear me out.
They could release a higher tier membership called Rune+ or something. Subscribers would get membership, RuneMetrics, a moderate amount of RuneCoins, access to the VIP world, 1 free exp boost, and double loyalty points.
They could fix the player model and continue updating and releasing cosmetic outfits from the Solomon Store. I think most people would agree that there is absolutely nothing wrong with cosmetics. But we do have a serious problem with the player model.
They could add double exp boosts. Each boost lasts 3 hours. You buy them with RuneCoins. Rune+ gets one free boost per month. Not stackable with Double Exp Weekend. Think about it for a second. No more gambling for exp. You actually get what you pay for.
These 3 things would be an honest replacement to Treasure Hunter. This is a standard MMO structure. It works.