There are many ways to handle a tree with little or no apparent taper. In this case a Ficus ‘Mystery’ was grown from a very straight and taperless cutting.

Fortunately there were several existing aerial roots that were brought down along the trunk and rooted into the ground. The aerial roots provide a visual thickening to the lower part of the trunk creating some taper that just does not exist. The roots coming off the aerials provide a nice surface flow that helps to visually stabilize the tree.

The area circled in red shows the straight cutting including a few aerial roots

The bonsai shows some nice taper provided by the aerial roots as well as nice surface rootage.