The ability to upgrade creatures and to train heroes in creature
specialties. For example, you could have a Spider King/Queen trait or path that
would allow the hero to recruit and train spiders.

Lvl 1 - Recruit Spiders

Lvl 2 - Spiders in stack with hero gain shoot Poison if they
are lvl 2 or make lvl 2

Lvl3 - Spiders in stack with hero gain web if they are lvl 3
or make lvl 3

Lvl4 - Spiders in stack with hero gain jump if they are lvl
3 or make lvl 3

These would be in addition to traits spiders start with.
This could be done for wolfs, bears, dragons etc..

I think this would add a lot a flavor to the game and make
the random creatures you pick up more valuable in the later part of the game.

Maybe limit these traits to one per nation, only one Spider
King/Queen for example.