I’m currently working on blendshapes in Zbrush, and i’m thinking about a plugin that could import / export fbx files, and convert back and forth morph/blendshapes into zbrush layers
This is a cool idea.
You can already make just one (or more) layers visible, and export that to an .obj file to get just that layer’s shape (it will be the entire model). Conversely, you can also create a new layer on your model, and import an obj file (with the same vertex and face layout) to get a new layer.
I am interested in why are you choosing FBX and how you plan to use this.
Fbx format handle Blendshapes/morth,
so 'im wishing for a plugin that could read Fbx format and convert each blendshape/morph and convert it into a 3d layer 
back and forth AT ONCE.
I’ve worked with model with a lot of (20+) blendshapes, i’m using 3d layers, and it’s boring to save each shape one by one to re import them into Maya, for ex…