It would be nice to have live preview of keystone and crop for live camera feed. This would allow using camera + ipad combo like large format view cameras are used. Code change for this might be quite small as basically this is just rotating the camera feed image plane in front of a perspective rendering camera, so zero performance impact. It would need sliders for H/V keystone, rotation and crop.
Current workaround
Currently I take shots with settings copied from the previous image and iterate until I am happy.