I don't think it will compete with the Arya as much as it will make it irrelevant. But I'm biased
As for tuning, I think it will be a less neutrally tuned headphone, based off the earcup design. So we may see this can does not have the same 3k hump that the LCD-5 has, but that could be replaced with a larger 5k dip than the LCD-5. This is all speculation, so don't quote me on this when the FR comes out to be contrary to my prediction.
I think the biggest difference between the LCD-5 and MM-500 will be speed and resolution, although I think the MM-500 will still be much more detailed than the OG LCD-X.
My biggest hope is that the MM-500 has a tuning and timbre of the LCD-R. That would be awesome, since it would not be bound by the Jot-A, which I did not like at all.