It would be very helpful if a callout had different style choices other than just the standard bubble to be drawn around the object. Some objects that require to be called out do no allow much space to clearly see a bubble around the object or there are several callouts being made in a small area.
One suggestion of style I believe would be useful is an Arrow. So instead of "circling" the item it can be pointed to and this would save much space and clarity in a drawing. This would also help in situation where a 4 sided bubble cannot encase an irregular object without it taking up too much space and encasing other objects not needed.