A friend of mine and I are doing several Holiday Bazaar's this year where we are selling RI items for Gingerbread House's and Cupcake Kits. She wants me to do a Dummy Cake so we can showcase our Cake work, but I am stummped! I do Birthdays, Baby Shower and Novelty Cakes. Look at my work you can see what I have done...I am thinking I need to do 3-D, but what do I make????
I did look at your work. I would go with Christmas theme (is it the right holiday time-wise?)
Easiest would be to steal ideas from your two cakes: Make a big Christmas tree - steal the look from your "landscaping" cake - the tree only and few mini cakes around it - again, steal the look from your "dots" cake but make it square, meaning present boxes.
If you are in time for Halloween - make a big pumpkin (round cake) or something similar. You will want to go only slightly above your "normal" level, but staying in the range of the cakes you normally do - meaning if you don't make carved 3D cakes (at all), don't make it for the display.
I know it's a Holiday Bazaar, but I find that at the holidays, most people don't order specialty cakes from me. It's mostly the big family events -showers, 1st birthdays, weddings, etc. What about a stacked wedding type cake - very elegant with white snowflakes to tie in with the season. Then maybe a detailed birthday cake to show your work. And, maybe you could make a sample of something small to give away... A packaged cookie (pumpkin, gingerbread or cinnamon remind me of holidays) with your business card?