The reason A6 allroad wasn't (isn't) very successful was partly because Audi demands too high a premium over the standard A6 car. If it were around 10-15% higher, it would be a much better seller, IMHO. The other reason is that in the US customers don't really like wagons because it carries with it the image of "soccer mom" car, hence they tend to choose the "more masculine" option in SUVs even when a regular Avant would do.
Incidentally, I like the idea of an A4 allroad if they price it reasonably. Why? it's sure to handle better than the Q5 (for example, a6 allroad is only about 50-60 kg heavier than a6 avant and with an equivalent engine is only on average 0.2 slower to 100 km/h) and I usually prefer a car shape than and suv shape. In fact, when the bumpers are in the same color, I think the current A6 allroad looks better than the regular A6 avant (admittedly, with its air suspension set to the lowest ground clearance).