The thing I miss most after moving from Sonar Platinum is mixing controls for my external synths. In Sonar we had a Track mixer for all tracks. I realise that S1 doesn't do that but, if you don't want MIDI tracks in the mixer console, how about a secondary mixer section or page for Volume/pan and other CCs. We'd need all the channels and tracks on one page, not separate pages for each MIDI channel. What I'd like to see is Volume slider (preferably with meter), pan, mute, solo etc.
And you should be able to mix multiple external instruments on the same page/console, e.g. I'd have several backing tracks on my Korg Liverpool with other tracks (on different MIDI channels) on my Yamaha MOX 6.
I was very surprised to find that S1 doesn't already allow me to mix external MIDI. It seems so advanced in other respects but way behind in this so I think this addition should be a priority because plenty of potential migrants from other DAWs will expect it and miss it. I am able to recommend S1 to others, for a number of its features and relative stability, but I have to point out ro them this current black hole!