Well this is hard. I don't much like any of the portrait shots, at least as rendered through a web browser. They look more they seem too contrasty and saturated (other than the linear one, which looks slightly corpse-like, perhaps because the colour balance is off).
For landscape / cityscape shots, a bit of excess colour saturation and lots of colour separation are what is needed.
This is interesting research, Anders. Makes you wonder why the likes of dxo choose to use the Adobe RGB space internally.
Makes you wonder whether it would not be easier to make the aim to be to produce an image that does not aim to be particularly pleasing but one that maximises the opportunities for customised enhancement. This is probably what your neutral rendition aims to do, or you could argue that the linear version does this.
As things stand, trying to tune the rendition by producing a bespoke camera profile may just be too difficult for users without your understanding of colour theory.
Skintones is much more than just the profile. It's the makeup, the lighting, the model, the white balance, and the colorimetric precision of the profile which as said is not a part of that series of experiments as I didn't really have a quality X1D target shot, just a less-than-optimal H5D-50c shot (but usually it doesn't make that big a difference, it's quite robust). I compared to Hassy's JPEG to check that it was not too off.
What I look at in this experiment is the effect of the tone operators, how they change the look in the linear reference shot. I put little value into if the reference shot looks good or not. I don't it's a task of the profile to change a real scene you don't like into a picture you like, and even quite heavily subjective profiles doesn't do that either, they mostly just enhance what's there.
I've noted that Adobe does what you suggest for
some camera profiles, that is make something that's not really much of a finished look, but more like a starting-point for creative post-processing. If I remember correctly the A7r-II and Pentax 645z stock profiles are like that, the stock profiles produce an undersaturated look, not particularly pleasing nor realistic, but low saturation profiles are more stable and can as such be better if you make heavy post-processing anyway.
Indeed the NTRO can be used for this too. It's agnostic about the tone curve, and a post-process-friendly profile would have quite low contrast, little shadow dip, and if DCP you'd disable black subtraction which you can do with a profile flag. It's also easy to control the overall saturation with the chroma scaling parameter which I've exposed in the GUI. With weaker contrast the NTRO result will be increasingly close to RGB-Lum to finally end up the same as linear.
I'm not too worried that it would be too difficult to make a bespoke profile. Or rather, I don't think the majority will want to do that. The result using defaults is intended to be really good, and I think it is. The typical purpose of using DCamProf is if you just want a profile that produces a neutral realistic but still attractive result, and this is what the default will provide.
But then there are those special users that want to tune stuff. You still don't need to know particulary much color theory to do so, it won't help you much. What will help you is a good monitor in good viewing conditions and a good eye for color. That last one is the most difficult. Most can say which one of A and B they prefer, but few can say why they do it, and even fewer how to modify the color to get it there. It's difficult to me too, so it's generally a bit of trial-and-error involved. The problem won't be knowledge of color science, but knowing what one wants, and having the eyes to tune it.
The discussions here become quite technical, as it's about the implementation. In the end I just put in the curve color space into the software and you don't get to choose it, as for the moment I don't see any reason to make it configurable. It may be that in the command line software though, which will allow for more experimentation than the GUI. In the GUI I try shave off things that is more a curiousity than important to configure for bespoke profiles.
That DxO is using AdobeRGB as working space I guess is just a design choice. If you indeed know that no user will want to have more saturated colors than AdobeRGB provides, then setting that as a fixed working space and compressing colors to that will simplify things. I'd think it's a bit small though, and with video standards becoming increasingly popular AdobeRGB may fall out of fashion in favour of colorspaces like Rec2020. So for the moment I prefer to provide an oversized colorspace with various gamut compression options.