A scenario whereby the following applies:

- Terrain variation

- Food appears, creatures need to eat food to not die, food is competed over by the group of creatures

- When a creature dies, food appears in it's place

- Different creatures can interact with each other (bones and joints become physical objects, being able to push each other)

- Mutations also occur in the layout of joints, muscles... of a creature

- Fitness measured by how long creature is alive

This would create a more realistic scenario, whereby creatures not only adapt how they use their body, but their body layout as well, to meet the challenges of not dying.

The mutation part would be confusing.

Sounds like a fun idea, but probably not viable. Just so much stuff to implement to make it work.