Any chances to support different targets such as SpyderChekr, Colorchecker 24 etc. and simple matrix profiles tweaked from a zero state by means of Hue, Sat and perhaps a Lightness fader and maintaining a neutral axis? For a LUT profile, I believe the Passport has too few patches and I prefer matrix profiles in general. What I can see now - there is too many controls and too little data to judge based upon. Anyway, a really good job done.
There's support for several targets built-in (SpyderChekr 24 is among the supported, not the 48 patch version though, have no data for that), and for the more expensive Repro edition also support for custom targets (then you can scan your 48 patch SpyderChekr), combining multiple targets and even loose patches not in a grid (for spot colors).
I can still guarantee that not all wishes will be fulfilled :-), one thing I've learnt about profiling is that many users have strong opinions on how things should be and they're often contradicting, so one have to choose which users to satisfy. Matrix-only profiles can be made, but it's not the top of the list concerning use case support. Central to the software is still the LUT-based functionality. In fact I don't really think it's that necessary to have a profile maker at all when making a matrix only profile, it's just nine values. It seems like most matrix-only users just want a matrix of those nine values which they can enter by hand. DCamProf command line is more than enough for those hardcore users, so spending lots of time optimizing for that use case when noone will buy it for that anyway seems a bit of a waste.
Concerning patch count I think that relatively few patches like in a CC24 is an advantage for matrix optimization. You just need a few good handles as reference to steer the optimizer. As a matrix is linear it can't do much with lots of patches anyway. There can be an advantage to have many patches to choose from I guess so you can pick exactly the color of the handles you want, but when tuning optimization you only need a few. If you have lots of patches when optimizing the matrix will just lock down to a good average fit and you can't affect it much, which is fine for automatic optimization but not so easy to work with when tuning manually to taste.
For a reproduction I indeed recommend a larger target like CCSG, for which there is built-in support. A large glossy target requires very careful made shooting setup to minimize glare issues though. For a general-purpose LUT profile I think the CC24 is still adequate. The thing is that the LUT should only do very minor adjustments, otherwise smoothness is at risk. The main use of the LUT in a general-purpose profile is to provide the tone reproduction operator, gamut compression and look adjustments, the colorimetric aspect is quite minor. It's even quite common to disable the colorimetric LUT and only use it for the other parts, that is use a matrix-only profile as the base. One can use larger targets though, and multi-target just as with the DCamProf advanced tutorial show, but my experience concerning general-purpose profiles is that it's more a curiosity than a real benefit. Taste may differ there though, and in this case the software does allow for both approaches. Just like DCamProf it's so feature rich that I as the developer can't really predict all use cases users will try. It shall be interesting to see.
Concerning the neutral axis I don't understand what the problem is. Neutral is simply set by the camera's white balance, right? The matrix doesn't affect that, at least not the forward matrix and that's the only matrix that is (optionally) tunable with Lumariver Profile Designer as that is the only thing that matters in 98% of the use cases. Should be said that both DCamProf and Lumariver Profile Designer is "limited" to make only "whitepoint preserving" matrices, that is matrices that don't shift the whitepoint. DNG profiles require such a matrix, but I think it's a general good design property so it cannot be turned off. If you use some other software or just manually enter matrix values you can indeed make matrices that does shift the neutrals away from the white balance setting.
(I still need to do some mundane testing, but it seems quite feasible to make a public release of version 1.0 in 2-3 weeks.)