Defense doesn't factor into WPA at all.
Probably we are not far from an extended WPA which does. We now have percentages reflecting the chance of any given fly ball being caught. Take the WPA that would have been added, if that fly ball had been a leadoff homer, apply the percentage chance as a multiplier (subtracted from 100%), and that's the defensive WPA for that play.
We Can Rebuild It. We Have The Technology.