First it would be huge so I never have to wonder if I can REALLY fit any more chickens. It would have electricity so I can run one of those big ass fans and a heated waterer for our stupid cold winters and some fun decorative lighting that my chickens won't care about but I will when I take a million pictures. A separate area for brooding. And if I am building my dream coop, it's going to be where roosters are allowed so I can let my hens brood their own. A run tall enough for me to walk in (my coop is but my run is only 5' and I am 5'6" so...) and in it one of those natural branch swingsets. And the outside will be a little house or a castle or something but with plants that deter flies and a little chicken garden that I won't care if they destroy because my real garden is somewhere else and this one is just for them.
And a coop cam so I can watch them when they aren't looking and see the funny silly things they do.