I wasn't really thinking of a system like of Heroes of Might and Magic.  Was thinking of developing a tech tree that would allow you to grow your own creatures.

For example, you've invested a little bit in Creature Tech, after killing a spider you are able to retrieve it's egg sack.  Bring it back to your city that has Creature Tech to enable the egg to hatch and be trained.  It then becomes a unit that can be controlled by a hero.  If you send the Spiders out on their own, there's a % chance they may go rogue, so keep them close.

I envision you would build a Monster Training arena that can be accessed by a Hero in town, getting sent out to obtain items, eggs, eye of newt, or whatever in order to keep the beastie factory moving along.