It is my opinion that many people do not pvp because they are afraid to lose their units and when they do PvP they will not use certain units for fear of loss. As a result Players become bored, exhaust existing content and leave the game.
If we were to provide a method where player loss of unit on death were less extreme I think that we would encourage more people to try pvp and as a result stay with the game. My proposal is to have a 50% chance of unit destruction on death in battle, and IF that unit is killed, to allow it to be resurrected for a cost in crowns. For elite units the cost in crowns should be 50% of the original cost in crowns + the level of the unit, for all other units, the cost would be 50% of unit level. Meaning if your level 20 foot knight died, it would cost 10 crowns to resurrect him. Given that there was only a 50% chance of him being destroyed in the first place, the real cost could be 10 crowns on average.
To make this easier from a development stand point, all killed units should be moved into an 11th army graveyard to await resurrection, or even to the bank to allow for purchase if easier.
In any case, I believe it would promote PvP, which would increase effective content, which should intern increase retention.
It may also generate revenue as some players who are less patient than others may actually buy crowns so that they can fight again...