Shapes, sizes and some form selections should be diverse in order to create visual interest. However, do not forfeit simplicity merely to create varied combinations.
Elements that do not provide improvement on the design can be omitted. Make sure to prioritize what is important and what is not in order to keep the design clean, neat and uncluttered (easier said than done ✅).
A simple, well but beautiful design is one that will be easier to maintain these days.
Everything that is placed in a design will carry a certain visual weight with it. Balance is the concept of ensuring the weight feels even throughout the plan.