Multi Instrument mode in Studio One 3 is fantastic for layering VI's (though I agree with other requests asking for more flexible channel selection in MI mode). This specific request, however, needs to begin with a comparison to Logic Pro X and Reaper.
The latest version of Logic (10.2) has a feature called Track Stacks, basically a folder combined with a summing bus for Instrument tracks. Any MIDI recorded to the folder track is fed to the member tracks equally. Instant layering! Faders and pans are controlled from the member tracks. So far, very similar to Studio One's Multi Instruments.
But Track Stacks go one further. You can also record MIDI data to the individual member tracks, which is fed to the summing bus as well. This allows you to add separate detail, e.g., harmony lines, counterpoint, thicker chords, etc., for the individual instruments in the folder group, making subsequent editing much easier than having to do it all on the folder track.
Reaper, while not offering something like a Track Stack per se, allows you to do the same thing by way of its virtually unlimited routing capabilities. In both cases, you can record and edit MIDI data on the folder members individually, in addition to the data on the "group master", for want of a better term.
So to focus my request:
1. Enable viewing, recording and editing of individual Muti Instrument tracks in Arrange view (and the related Editor, of course).
I believe it's already possible to create Inserts and Sends on individual Multi Instrument tracks, so this added feature would make MI mode even more productive.
Please vote this idea up if you like it.