This was actually asked about in another forum but I was teaking my system and found it would be great to have. I would like a digital out and digital in - similar to the old fashion 'processor loop' functionality. Here's how it would work:
The digital out would be after the input selection, but prior to any processing. It would remain the the stock format, PCM, Bitstream, etc.
It could then be used with external digital processors and return in the same format (or converted) to the digital in. Then the 990 would do whatever it needed in terms of processing, volume, etc.
It could be bypassed with a switch just like the old processor loops.

The necessity for this came when using my DEQ2496 to do some equalization on my mains. The DEQ has digital in/out so why not keep everything in the digital domain until the last and let the 990 do it.

Also I would like to see AES/EBU digital I/O, just because it seems a lot of pro equipment (like the DEQ) is being used in HT these days.