You can sub-mix already, by using the older mixers. I tend to use the same colour when grouping channels together. It's a simple idea that's easy to do. I do see where you're going, which is towards something like a Neve-in-a-Box.
Ability to fold racks? Can't you do this with Combinators? Perhaps you've not made it clear what you're looking for.
I'd go with a new version of the Combinator, generally extend its features. More CV would make sense with all the RE Utilities there are now. How about a dedicated area? That would go for Audio routing too.
This is where MIDI could also come into play, perhaps. How would you implement it?
Subtractor's about due for a new version. Maybe it can have Stereo Outs. Would any new or improved instrument be another RE? Generally, the older instruments and effects could be reviewed. Mind you, there are so many things coming through with RE's now.
RE's into groups within the RE Soundbank would parallel how things are in the Shop.
MIDI and MP3 implementation could be general improvement features.
Reason for iPad? That's a separate product, surely.
Off the top of my head..... meaning, I could do with more thought on this. Aren't we already supplying ideas anyway, through these forums? No doubt I'll forget..... whatever it was I was..... My mind is going like HAL's.