I loved the Chronicles of Narnia! It would be a fun mod, but would definately work better for RTW. I myself don't have RTW (my computer doesn't support it) but I know the limits in MTW and some things wouldn't work out. To have an army made up predominately of animals, you would need more sound effects, and as it is now the only sfx we have are for Infantry, Cavalry, and Camels. Thus you might be able to have Infantry, Cavalry, and another sound (I don't remember camels in narnia,) such as Elephants, Leopards, or Dogs, but it would be a TON of work doing the map and graphics. I wish I had the time to work on this idea!
Justin