Complications either due to time or complexity. For instance, the Magma guys would allow you to stand on top of them in lava areas. However, they scrapped lava areas after they realized it just didn’t work too well. A lot of this is mentioned in the development of Evolve thread.
Maybe they could add a few of these things in big updates
I would love to see them. I want to vent my frustrations on Cryosaurs
There are always DLC maps, some of these creatures could be included then.