If members begin trading good through the community, create a marketplace for it.
If members begin sharing reviews of products/service in the community, create a review section for them.
If members begin training each other on a topic, create a peer to peer training area for it.
If members begin gossiping about celebs, create a gossip area.
If members begin sharing their best photos, resources, stories, experiences, create specific areas for them too.
The goal is to respond to what members do. Find out what's popular and build activities around them.
Better yet, you can try each of the above to see what takes off.