Designing elements such that, when those elements interact with each other, new elements emerge, even though they were not intentionally added by their designers.
By definition, a designer cannot design emergent elements. They can, however, design elements that can interact with each other in surprising, emergent ways.
Examples of Emergent Design
In Tears of the Kingdom, designers intentionally designed ultra hand, a feature which allows players to glue objects together to build contraptions. They also designed the portable pot, an object that allows players to cook food. They had the problem, that they needed the pot to always be flat, even when it is placed on an uneven surface, so they designed the pot with a spring, so that it will flexibly always stay flat. Then a new element emerged. Players began to use the portable pot as a suspension system for cars, because the spring perfectly met that use case.