We could not see the whole tree in your photo. As the tree matures, this area may become a weak spot and become a hazard. You may eventually have to replace it. If the tree is in good health and not a hazard to any structures, you can let the tree be. We are not on site and cannot offer more recommendations.
You can contact several certified arborists for an on-site visit regarding the health of the tree and the best way to proceed. http://www.treesaregood.org/