It is not the Product Owner's sole responsibility to build the product backlog on an on-going basis. As your Initiative evolves, your team will gain incredible knowledge of the product your are building. They become an invaluable asset to the continuing development of your backlog. 

The Product Owner should encourage the team to suggest new features to be released. He should also strategically watch what the competition is doing.

It is recommended to maintain your backlog with members of the team, once every other iteration. Take this opportunity to clarify certain user-stories, re-estimate them, question their priority, business value and level of importance.

• Ask your end-users what they would like to add to your products. End-users are fantastic user generators that do not have the blinders that you have when it comes to your product! Take advantage of that. Your end-users will find it rewarding thus increasing their loyalty to your product. Win-win!
• Ask partners (Affiliates, 3rd parties, integrators, etc) what they'd want to see in your product. These are often powerful channel generators that can significantly help you grow. Why not make them happy?