I can't say anything about subD support, but I can say that the mesh optimizer will get much better sooner than anything else. Specifically, a new simplification mode that attempts to optimize for "quality" relative to the kTri target you specify, including a quality mask you can supply to show areas that need to retain higher detail.
Yes, please consider adding the ability to export as a Pixar SubD surface, or some other form of loss-less 3D format such as NURBS. SubD is much better than a higher res version of the terrain, and requires potentially less from your GPU & CPU. If not these, then at least incorporate a smarter mesh optimizer, which reduces the polycount where there is little detail, and increases the polycount where there is need.
As it stands now, the Mesh Exporter has a lot of room for improvement. It doesn't optimize the mesh topology in any way before exporting. The current exporter uses a uniform distribution of quads & tris, regardless of topology or detail. A smart exporter would incorporate some form of optimization. The current optimization is too simplistic.
I'm a big fan of using subdivision on terrain in games for increasing their resolution and adding a certain feel to the terrain. Although it's more space-efficient to do it on load, for small terrains it'd be much easier for me if World Machine supported subdivision upon export. Please consider adding it!