Other than CMOS, Pentax and Leica have already delivered what you asked for. A dslr handling semi-medium format camera that has close to the functionality of a modern 35mm dslr, with excellent lcds, in camera jpeg processing, faster response than the typical medium format camera.
They may be missing cmos and true multi point autofocus, but those cameras are as dslr like as it gets, actually they are dslrs.
They also cover both ends of the spectrum, a 9 grand body (Pentax) and a $28,000 body (leica).
But if you really want what I think you want call fuji, because they dipped their toe in the water a long time ago.
The only problem with this back was Fuji didn't go further with development, which means they didn't make a smaller 680 camera or a larger back, but let's be realistic;
If the market was there cash in hand for a 6x8 digital back or a cmos digital back or a fast 10 fps, 10,000 iso digital back, I don't have any doubt that some manufacturer would not fill that roll.
In fact the market seems to have gone the other way, with less expensive, more megapixels, almost 2 year and gone cameras. Nikon positioned their new 800 as their studio professional camera and Sony, Canon, all have gone 24 something mpx for under $4,000, with less than robust bodies.
Buying a camera, even for professionals, is sometimes like buying a car, a house or a suit. You don't always buy what you need, you buy what you want and most importantly can afford.
If you can afford an expensive niche camera, car, house, suit, then go for it, if you can't you buy something more pedestrian and main stream, but I don't believe with still or motion cameras we're ever going to go larger than 645.
I made a point on this forum years ago that I felt 24x36 was the new medium format, 645 was large format and aps c or 4:3 were the cameras that filled the general population 35mm roll and of course on this side of the forum screamed like it as blasphemy that anything less than almost 645 was even considered medium format, though that's the way the world seems to be going.
Apple has given up on 17" powerbooks, and large desktops and when I travel by plane, instead of everyone viewing a laptop they're looking at an ipad. Electronic devices are getting smaller, some delivering big results, but I don't see anyone going bigger in just about anything from cars to cameras.
Given that, I just don't think it's a one size fits all world. In the typical shoot day we use different formats and I find nothing wrong with that other than we have to carry a lot of camera cases and lens sets, but that's just the way it is today, at least for my work.
Recently the only news that even half way excites me is the new Arri LED fresnels. 750 watts, low draw, daylight balanced for under 3 grand.
http://www.arri.com/l-series/performance.htmlIMO
BC