Every comp should include the location of the anchor point, which will be used in the parent.
Currently the anchor point on comps and templates can only be set to an outside edge or the center. Instead provide an actual field in the composition settings where you can enter the anchor point. Clearly for any layer you can move the anchor point to a specific location, so this isn't a foreign concept. I've been creating templates and the most annoying thing is that in a complex template I can't set the most useful anchor point. The user of the template must either deal with the fact that they won't be able to rotate/animate/etc. the template usefully unless it is fully balanced, or they need to figure out where the anchor point should be every time they use the template. Instead I want to provide the proper anchor point and avoid all doubt (not only speeding up their processes, but mine as well). You could keep the existing arrow control and allow them to toggle between that and providing the proper anchor point, but I would just have the arrow control update the field and then allow them to change it.