Well, the simplest thing would be a way to skin garage doors with non-solid blocks such as fences. But yeah, an alternate garage-door-type block that had a fence-type model that could be skinned with whatever block you wanted to would be awesome. You could have a portcullis made of wooden beams, iron, stone, whatever took your fancy. Or to be really disturbing, skin it with Thaumcraft flesh blocks....
I like that idea. I wonder how good a portcullis made of glowstone would look. Alternatively, I could go for the seemingly-impossible sand portcullis.
So, a new style of garage doors that looks like stacked fences?
Sorta. I guess it just wouldn't be very easy to automatically fetch the "side view" of non-solid blocks and auto-apply those to a garage door, even more so because we are talking about a specific BlockState of those non-solid block here.
Maybe it would be easiest to just create a new model Json mapping the vanilla texture of Vanilla Wooden Fences, Nether Brick Fences, Iron Bars and what was it, Caermaster said again? Vines? to a "block" of garage door.
And then, of course, enabling these blocks as a suitable "skin" for garage doors.
I want to know Mineshopper, would you want to use Java model in your mod for 1.12.2?
I mean dynamic java models like the ones I use with my mod?
It's not so hard to do and it will help you to get the same result you was having in 1.6.4/1.7.10.
Just ask me in private message if you want so.