I built a nice little audio streamer last week, using a RPi 3, a 7" display, the case from oneninedesign, and a Digi+ board.
There were however one problem: The display needs poower.
The display can be powered in two ways, either using an extra USB cable, or using two pins from the GPIO connector.
However, the box did not have space for any USB cable for the display, and the Digi+ board uses the GPIO connector, so it is not available.
Luckily I still have my trusty old soldering iron, so I fixed it that way, but not everyone may have that option.
Thus, if you add a small connector to the P3 connector, you could add a power cable which may be used to power the 7" display.
I imagine the same idea would work for the HifiBerry Dac as well.
Just a thought.