I'm a fan of Taylor Koa 12-strings. Wonder how a Koa DN would work out
I've pondered that myself; but I already have a K66ce, so I kind-of wanted to do a different wood than koa (or for that matter, maple, as in my 656ce).
But since it's likely to be a 12-string Dreadnought (DN), which is otherwise unavailable; and if they have a particularly striking set of koa to pair with a European Spruce top, then perhaps I'll go that way...?
a deep body X10 (a 12'er would be a X50), macassar ebony back & sides, if it's available "off-menu",
sinker redwood (hi $) or adirondack top with adi bracing would complement your 2 X16 12'ers well. imho 
Macassar also crossed my mind. Macassar is also the top wood on my 12-string T5. Again, I kind-of wanted to do another wood than is on my existing 12-strings; although in the case of the T5's, the top wood seems to 'color' rather than 'supply' the basic acoustic sound of the guitar.
Also, I hadn't really even been considering another top wood, as I went into this mental exercise by building the guitar around a particular top wood (European Spruce). I'm working with Joe at Empire Music on this, and he'd expressly mentioned to me that he was going to Taylor in March to build some customs using European Spruce. Already owning a BR-V and a 614ce 2013 Spring Limited, both with European Spruce tops, I found that I preferred it to the typically-used Sitka Spruce; and even to Adirondak Spruce, which I'd 'test driven' on a Custom DN (Adirondak/Mahogany) that Joe had in his shop about 8 months ago (frankly, I preferred the sound of the standard, Engelmann-Spruce-topped 510 that Joe had at the same time, which is what just let me to have him order me in a 510ce rather than go for that Custom DN). So it occurred to me that European Spruce might lend itself really well to a 12-string guitar, and the concept took off from there. Given that European Spruce was the starting point, I'd sort-of eliminated Macassar simply on the basis that I didn't necessarily want to seemingly copy the 2013 Spring Limited guitars. But that was before I'd moved away from the idea of doing the 12-string as the 'standard' Grand Symphony body to a Dreadnought; so, given that the intent is to do a different body style as a 12-string, perhaps I should reconsider Macassar as well...?
As another aside, I tried another Custom DN that Joe briefly had at his shop during a Road Show: one with a 'Sinker' top. It was a beautiful-looking guitar; but frankly, I was not impressed at
all with that guitar's sound. Neither was my brother. And to tell the truth, I got the distinct impression that Joe didn't think that much of it either. I can't remember the back/side wood on it, but I'm pretty sure it wasn't Macassar, so it's probably not a fair comparison on my part.
As yet
another aside, Joe had a Custom GS-12 up at his shop for a while: 'Sinker' top over Rosewood back/sides. The more I played that, the more I liked it. Fortunately for my wallet, someone else bought it before I got too attached to it. On the whole, though, I still think I preferred the sound of my 656ce and K66ce to that Custom GS-12...
All good suggestions, guys! Keep 'em comin'!
