If it was shot down in their airspace, nothing. We lost a drone, nobody died. Are we going to kill people over a drone? Avenging Northrop-Grumman? We're the ones escalating tensions over there, again. Hell, the reason we're pissed at Iran is that they got sick of the despot we installed and they eventually overthrew him. We then refused to return the despot to their country so they took hostages. We started this fight long ago and now we're back to do it again.
If you want to do something about it then grab your gun and head over there. "We" have caused enough problems over there, you go have some fun.