For me, the ability to switch the polarity of a mix would be ideal.
I feel that being able to split an output and then phase reverse one socket is a little overkill, if not dangerous in the wrong hands. And surely if you really needed to do the latter there are a million ways to do it! Like for examlple running an additional matrix of main L – with it’s polarity inverted.
Also as a side note my t-80 is doing monitors on one stage at leeds festival next weekend. If you’d like me to take any pictures for your press people please drop me an email; bill (at) blackmagicaudio (dot) co (dot) uk