I think cosmetic things would be really great, like different outfits for the Keeper. Also, if someone finishes a particular quest, then you could buy a cosmetic item specifically from that quest. So if a hero wins a quest where they have to go kill vampires, you get the option of mounting a vampire head in your tavern for a small amount of gold. It'd be a nice way of remembering successful quests.
There could also be more technical things like buying more tables to support more heroes in the tavern. I really like the idea of having to get some of them drunk or eavesdropping on them! If you introduce a drinking system, maybe you have different heroes who prefer different drinks, like some prefer a beer or others and ale, and if you can get their drink right they tell you more about themselves. Hope this helps :)