I'm using FC for a week or so now and I'm not familiar with the source code yet good enough to make the requested features, thus I only tried making saner default looks for the naviCube. Now it seems to me that without things being configurable it will be hard to make everyone happy and agree on something.
It's good to know other people's needs though. However I might not be able to provide you the missing features... at least for now, not sure if ever.
It seems to me that the adjustable widget size (and transparency/contrast?) might be the most important missing piece especially for High DPI screens. I can Image that the Cube may render to small on those, making it unusable or depending on user's eyesight it may seem to light or transparent. It shouldn't be that hard to implement these I think. The second item would probably be those configurable labels. But this one can be tricky depending on how far we want to go with it and have consistent naming all over the place (incl. translation support). Maybe this one could be broken down into a few smaller, incremental steps: 1st - translatable and/or consistent labels with other modules, 2nd - option for custom labels, 3rd - option for custom global world directions (used by cube labels and other modules). Regarding additional buttons for the multitude of available view commands - I would rather stick to the current single menu button to keep it clean and tidy.
I'll see if I can invest more time into this. But again I won't promise anything as I might just prefer using the tool rather than making it fit everyone's visual taste and needs.