I find it very helpful to pull out various fabrics, trims & embellishments and layout how it will look. It would be wonderful if all the components were just what I was looking for (I usually have particulars in mind such as colour scheme) but that is rarely the case. While I have an extensive collection in my studio, it seems that all the bits that I may wish are never there (for example: not the correct shade, colour, type of trim, etc). My choices are quite simple, check the stores to see if the exact item is available (also extremely rare)...or tweak my vision to work with what I already have. I found that being flexible & allowing for variations often is best course of action - new design ideas are born! Visualizing the final product & taking into consideration what is available (either already in my studio or at the store), makes it easier to determine next steps - how difficult will be to sew it.
Although all the ideas will look great, difficulty of construction determines which design will be made into a pattern. I walk through the construction process in my mind (how will it be made) - what steps will be needed, how will it be to sew by machine (will the thickness fit under the pressure foot, can the area be accessed to be sewn by machine, etc), where/when will embellishments be added, what finishing will be needed and what/how much must be done by hand.
Designs that turn out to be too complex, make it difficult convey the instructions needed about how to make it, as well excessive explanation/photos/diagrams affect the size of the final pattern PDF thus making delivery problematic (via download or by email as an attachment). Really complex designs are restricted to handmade & custom made only and will not be turned into patterns.
The final embellishment pieces will now be drafted if they need to be created from scratch.