[Suggestion] Capture Enemy Units
I think that it should be made possible to capture enemy units. Choosing this option over killing would probably cause there to be some sort of attack/defense penalty due to the fact that you are trying not to kill them. This would mainly only be useful for negotiations, where you have captured some (or all) of the enemy's heirs, or to "tame" wild monsters. Also for the empires (later on of course), if some of their magics require sacrifices, it would be better to use enemies for sacrifices rather than your own (assuming that the sacrifices would have to come from units).