The tree has obvious positive traits that promise to make it into a really good bonsai. The problem is, as I see it, the tree has been neglected for several years. I find it difficult to believe that a tree with all of the qualities of this tree has not been taken care of. Also; the soil looks like the roots have been neglected and its need for repotting probably out weighs its need for anything else. After the repot I think all that you will need is some ramification work and a bit of carefully placed wire. Take it easy on the wire, you don't want to make this tree look contrived.