NEC PA302.
It's a professional, color-correct, pixel fault class 1 display. It costs two grand (incl. tax) here. Maybe it's a bit cheaper in the US.
There are also various 38" displays that do 3840x1600. I'd have to see how these work in practice, though.
I think the 2014 Mini can drive those.
Actually, I could get the PA322UHD-BK for 2.5 grand, which is the 4K version.
I just like nice displays.
Currently using the old LP3065 from my boss. It's not a great display, but at least I got it for free.
Power-usage is absurd. It uses 10 times the amount of energy than the Mini, its maximum power usage is almost 180W - the PA302 is at 59W.
So, a Mini (or any headless Mac) and one of these displays, that would be great. I don't really need 3D or AR. Just a nice display for OmniGraffle, Sublime, Pages, Safari.
Ideally, the future headless Mac could drive at least three displays.