what about adding a new animal peice? Maybe a croc or snake? the croc could go on land or water and be able to kill anything weaker than a lion. perhaps also make it so that it can only attack while in the water or when coming out of the water. if already on land it can't attack.
A snake could be like the crock only it could be killed by any animal. in addition it could possibly kill any other animal(poison) in return. though, perhaps not the elephant. you know, to keep it as the strongest piece in the game.