I don't know how to create mods myself and would like to ask you to create such a mod. (If anyone can, please teach me how to create mods.)
Here are a couple of new Castles:
First Castle:
1) Forest
Creatures in this Castle:
1. Forest Fairy --> Lake Fairy.
Fairies can be hired in the Forest
2. Tigers --> Tiger Riders

3. Bears --> Ancient Bears (Special ability: Enemy does not retaliate after attack)
Bears can be hired in a den
4. Lizardmen --> Lizard Warriors
5. Druids (Special ability: Can summon bears) --> Forest Mages
6. Forest Giant --> Mountain Giant (Special ability: Can stun the enemy when attacking)
7. Forest Dragon --> Great Dragon
Second Castle:
2) Underground Kingdom
Creatures in this Castle:
1. Underground Lizards --> Lizard Archers
2. Stone Warriors --> Stone Archers
3. Underground Birds --> Underground Griffins
4. Underground Elves --> Thieves
5. Rangers --> Assassins
6. Underground Gnolls --> Marauders
7. Arshi (Huge creatures on two legs with a face resembling a dog's muzzle) --> Arshi Kings (In golden armor, holding a large spear in their paws)
Third Castle:
3) (I haven't come up with a name yet. This Castle is "inhabited" by Gnomes)
1. Gnomes --> Miners
2. Gnome Mages --> Rune Casters
3. Battle Gnomes --> Bear Riders
4. Gnome Archers (Shoot lightning) --> Lightning Lords (Shoot twice)
5. Apprentice Mages --> Sages (Casts a random spell on friendly creatures)
6. Defenders --> Conquerors
7. Lava Dragons --> Dragon Riders

Here are the changes I want to make:
In Necropolis, I want to replace Skeleton Warriors with Skeleton Archers. And instead of Death Knights, make Death Angels.
In Rampart, replace Battle Unicorns with Unicorn Riders.
In Inferno, replace Gogs with Cursed Unicorns.
I also want to add a couple of creatures (that cannot be hired in cities):
1. Necromancers
2. Dragon Apprentices