Maybe you could make take some ideas from Whitelight's ideas for Minecraft (link to one of the key videos on the topic). They tend to highlight the needs for this genre of games to balance the wants of the intrinsic and extrinsic. Mentioned specifically in his videos is that dynamic systems can be used to give purpose to actions normally only taken by the intrinsically-minded, such as building large fortresses that, with dynamic systems, can be used by NPCs and thus provide protection and attract useful NPCs. Some of these ideas definitely are very difficult to implement, but maybe you could make scaled-down versions.