Generating design ideas is the easy part. It's a lot like play, as others have noticed.
It felt like play when I was composing the brooch mockups shown above. There are no rules at this stage of the game. It's best to let intuition take over.
Rules enter the process later, when it comes time to bring the ideas to fruition and the elements together, forming a single structure with an unobtrusive supporting framework and a pin stem situated as effectively as possible.
Flexibility is crucial to the creative process. I've learned to let the process unfold on its own terms, knowing that the end result will always be better.
In this way, mockups are like questions or hypotheses, to be tested and perfected as the work proceeds. Or like a term paper. No matter that you finely hone your outline - new ideas slip in as you move from page to page. Accommodating them makes the whole paper better.
At this point I can't say which of the brooch mockups above will end in a piece that differs significantly from my initial concept.
And that's ok with me. It's part of what I love about doing new work.