I think a cool idea might be a defensive only unit that when it is attacked, it does massive amounts of damage to all units at that location, including its own side. This would balance itself in a way, by making it a great new "ninja" unit, that if its all alone, does damage to the attacking units only and dies itself, but when attacked with other units at that location as well, it kills alot of the defending units as well. It makes it a risk to keep, but gives a new flavor to the typical fleet-saving.
Obviously an idea to be tweaked, but a new idea
Obviously an idea to be tweaked, but a new idea