here is something really simple that could be realized in the OS alone.
voices 1,2 and 3 go to the matrixbrute like they do now, for instance in paraphonic mode. voices (say) 4,5 and 6 when you hit these notes on the keyboard, are routed to 3 of the cv outs, for instance 3 of the 4 assignable routings in the matrix.
instad of arturia developing an expander, you develop your own in the form of a small eurorack. all you need is oscillators and maybe a small mixer. the oscillators are controlled via the overspilled cv from the matrix, and their audio can come back via the external input where they can get processed through the filter and vca just like the internal vco's. big plus is that as a user you can select your own sounds (a matrixbrute with wavetables) and that all Arturia has to do is create some extra code in the OS.