I mentioned this build idea in passing previously as a comment in
https://www.element14.com/community/docs/DOC-80619#comment-81479
however I thought I'd submit it as a build idea as I really could have done with it this week 
I often find myself in situations where I need to merge multiple headphone/line outputs into a single pair of headphones for audio monitoring purposes as well as having a visual indication of level activity.
One scenario is as a video camera operator where not only do I have to monitor the microphones connected to the camera but also to headphones from a separate intercom system, and to line/headphones from a camera mounted encoder. Juggling headphones and headsets all day is just a pain. In this case some sort of a compact belt back mixer or one that could be clipped onto a tripod leg would be good, allowing switching or mixing between devices. Having selectable ducking on the intercom input would be useful.
Another scenario is live webcasting where the headphones are normally connected to a portable vision mixer, however I also need to be able to monitor the line/headphone output of laptops, mobile devices, IPTV set-top boxes etc. to verify the audio is present and good, as well as an intercom system, and as of this week a Bluetooth enabled walkie talkie radio system. As well as being able to solo and mix sources it would be good to have visual monitoring on all input channels along with a selectable ducking option on the intercom / radio inputs. As space is always limited then a compact desktop unit would be good.
As each scenario is likely to have different requirements I was thinking of some sort of a modular channel strip type design which could be easily adapted to the user requirements as required. e.g. 3D print a longer case to add in the additional channels.
The closest off-the shelf product which I've seen is the likes of the RDL RM-MP12A
RM-MP12A ‐ Audio Monitor Panel - 12 Line or Speaker Inputs
however it lacks certain features like stereo input monitoring, ducking, Bluetooth. It however does have selectable input levels, ballistic LED level indication per channel, etc. so part of the way there.
To get all the features in an off-the-shelf product then you typically end up with a large mixing console with a whole load of additional features which aren't required for what is basically a simple monitoring requirement.