I would first ask what you drive most of the time? Unless you are a Training barn, you might want to just keep the harness you use most out on the “wall” hooks. It just collects a lot of dirt stored out of the harness room, synthetic or leather, so you need to clean it off before using it or keep it looking nice.
I would absolutely keep any show item put away to save getting dirty or marked up. Also easier to keep the metals clean if harness is stored, for show preparations. Harness stored correctly on proper racks or in trunks, will not have the full weight pulling down on it, which can enlarge holes, puts stress on stitching and leather over time. Especially hard on bridles with the extra weight of bits pulling down.
There are inexpensive harness racks, sets of racks, for hanging your harnesses on when they are off the equines. You may want to make your own racks. I want a harness rack for collars that has a gentle radius, so the entire inside of a upside-down collar is supported, not just where a narrow hook passes thru it. A skinny hook can pull the curve into a sharp point on the collar leather. Same thing with a breastcollar or breeching, both made to curve around the chest and rump, no sharp folds. Harness saddle needs a bit of an A-frame, so tree inside is following the same shape it is made with. My bridles that are hung, also need a bracket or device that gives the crown the needed curve that a horse head has.
I don’t want any of my harness reins, traces, pieces/parts to have a sharp fold in them. So they are often GENTLY curved in being hung up, using the same bracket as the breastcollar or bridles. We usually hang the traces from one end, no bend or fold in them at all. We have different singletree attachments on various vehicles, so having the extra traces hung at hand, saves time when we need to change them out.
We have the Zilco harness hung out for daily work most of the time. This would be 4 sides, could be used for a Pair or the Four. Takes up most of our only vacant wall space. Getting any other harness used, requires us to haul it in and out while getting the horses comfortable wearing it, fitted right, before competitions.
Ideal Harness sells brackets:
http://www.idealharness.com/product_info.php?cPath=20&products_id=3400
Driving Essentials has those same brackets and the Zilco Harness bracket with all the hangers on one piece, that a lot of folks have said good stuff about.
http://www.drivingessentials.com/harness_racks.php
Other folks have other bracket designs that can work well too, you just have to shop around to see them. Rounds cut from fence posts or small tree trunks, could give you that good radius for curves needed to hold breeching and breastcollars, bridles. You could paint or stain them to keep the wood dry and go with your barn decor. Making those rounds deeper, front to back, could let you put a couple harnesses on the same holder, save some wall space for you.
We harness in the aisle, hitch the vehicle there too. Nice not to have to carry things far. You are protected from the wind or weather. Our aisle is a straight length, front to back, so we can drive on out, back inside to unhitch.