How to create sophisticated legend patches for QGIS

Legend patches are abstract representations of the features in your map. It is a different level of abstraction from the one you used to draw your features because it represents all of them with a single shape that will use the style you assigned for your layer. Therefore you want to be as minimalistic as possible, but minimalistic does not mean to be simplistic, your patch could be as complex as you wish, but bare in mind that it will represent a set of features that share common characteristics

Think about a river, how would you represent it abstractly? Let’s suppose that is the Mississippi. Or, let’s imagine that is the Nile. Would you represent them the same way? What about a natural reservoir that has mixed areas, Imagine this reservoir with ponds, lakes and pools, a creek and a small waterfall pouring water on a big river; then you have forestry, grassy and wetland areas. You want to represent everything differently. What would be the difference between a lake, a pond and a pool? How would you represent each feature?