Obviously 100% backwards-compatible, MSX-AUDIO included (not sure about SCC being from Konami). Since he is considering upgrading the CPU speed I think he should also consider the OPL4 and V9990 extensions. In my opinion they are a natural leap forward and already well established among us.
If possible, I would love a hires mode (640x480, 800x600 or even 1024x768). This may sound somewhat heretic, but with the emergence of network interfaces, such resolutions would help us a lot in browsing the internet - and, who knows, using MRC directly from our MSX'es. 
Hi Konamiman, your story is truly interesting! Mr Nishi seems to really be a kind person 
. To me it's quite normal that he pays attention -at least- to people like you, who really deserve it due to their involvement. Your state of mind may look like his own, all the more since he's working agin on a new MSX system. I know however our community may not be typical to future users/buyers who will belong to modern times Ok, we do too but are not "a market"... Such a market really is to be defined, challenging!!
Just one question of curiosity, you said Mr Nishi is now aware of your creations, or such projects as MSX VR Virtualizer... but does he know about SymbOS? It really does something previous MSXes was supposed "not to be unable of" in past times,and that was clearly missing when 16/32bits competitors came up. SymbOS, trans-generational and multi-platform compliant, is quite a major phenomenon, isn't it? Was it mentionned during your interview and do you know M. Nishi's views on it?... I'm so curious
Thank you! So beautiful story!