Hmm... I'd say the perfect Port storage cabinet is one that's in my house!
I have three of these things, and easily the best one is my Eurocave. So working from that, there are a few things I'd do to improve it.
I find that my Eurocave 283 is too small. The "283" will allegedly hold 260 bottles when properly configured, but I filled mine will rolling shelves so in reality it only holds 9 x 12 + 1 x 11 + 2 x 22 + magnum shelf (holds 9) for a capacity of 163 bottles and 9 magnums. Why the varying numbers?
The rolling shelves are perfect and hold 12 bottles each. They roll smoothly, hold the bottles in place even when not full, and don't waste any space. The unit is exceptionally well-designed for these shelves.
The single wooden shelf is structural, but poorly designed in that it only holds 11 bottles - 6 in front and 5 in back. It should be designed to hold 12 as well. Also, the wooden shelves and the metal shelves don't mix well and cause wasted space when used in the same cabinet together. To fix that, there needs to be an option for a metal structural shelf that integrates with the metal rolling shelves. I'm sort of assuming that the wooden structural shelf integrates fine with other wooden shelves.
The pair of 22-bottle shelves that I have are wood but non-structural, and space poorly with the metal rolling shelves so I stack bottles on them to fit 22. Ideally the unit would be an inch or two higher so that those 2 non-structural wooden shelves could be replaced by 4 metal rolling shelves.
The magnum shelf is the compressor shelf at the bottom of the unit and is about as well-designed as it can be, but could still use another half inch of clearance so that the magnums would stack more easily - right now they're pretty tightly wedged in. This need might go away if the two non-structural wood shelves were replaced by metal rolling shelves as the metal shelves might give the magnum shelf that little bit of extra clearance that it needs.
Why rolling shelves? Well if you're storing your Port in a refrigerator, it is probably because it isn't in cases and is an assortment of bottles. If you're at all like me, you probably have lots of odds & ends and want to be able to pull out a shelf and see what's on it. Even if I kept a spreadsheet of where everything is located in my refrigerators, I'd still want the ability to pull out a shelf and show the bottles to others. Thus the need for the rolling shelves.
The last change I'd make is an external temperature display. C'mon... an LCD or LED display is cheap and it could just be embedded in the door. I have to open the door to check the temp on mine.
That's it. You can get the Eurocave with a smoked glass door, but I don't see the need. It's expensive and at least for me isn't an improvement. I guess it would let me see the temp without opening the door, but is that really worth the $700 they want for a glass door?
You could argue for a larger capacity, but the height and weight of my current unit are about right. If I needed more capacity, I'd just buy two and sit them side-by-side. I wouldn't argue against making it a little taller so that it could hold 15 or 16 rolling shelves, but that would make it a non-standard size when compared to regular refrigerators.
My second unit pales in comparison to my Eurocave. But then again it was 1/4 the cost. I keep my VPs in the Eurocave and my Colheitas in the second unit. The second unit's shelves are wire, don't roll, don't hold odd-shaped bottles well, and don't fit magnums at all. It doesn't have a structural shelf like the Eurocave, so it's extremely sensitive to the level of the floor. In my case the door closes and seals okay but isn't entirely square. The second unit is also significantly noisier. Luckily they're both in a separate room with a door so you can't hear it in the rest of the house.
My third unit is a Danby 56 and is simply too small. It was the first one I bought and I didn't realize at the time that a fridge that holds 40-ish bottles is basically useless for storing Port. Right now it's being used to store Douro wines, but I can see the day when those are going to get kicked out to make room for more Port.

The Danby's shelves slide (they don't have rollers, they just slide out) but are made of some thin wood and so they're getting warped from the weight of 12 bottles. I'd rank it slightly about the second unit, but nowhere near the Eurocave.