Consider implementing the mechanics of what I call Garden of Eden vs. Eat or Be Eaten.
With the bounty of Eden nature has the resources to experiment, When scarcity or harsh environments predominate economy of form is the priority of need. Don't Starve seems to be doing this, but more as an element of balancing difficulty. Using another persons suggestions of different spider species as an example, they could start appearing under different conditions, when certain foods are present, during certain seasons. Another for instance would be existing resources for sails in Shipwrecked could make superior sails when their growing conditions reach optimum, or seaweed could bloom during the summer and create a run of fish. Plants or mobs might actually develop rare or flashy strains that appear under special conditions. Then again, some could be dodo birds.
Just an idea.