I have two suggestions for improving the workflow when assigning sidechaining within S1.
The first one is basically a reverse way of doing it, but one that actually makes a lot of sense.
Every plugin that can have a sidechain input assigned has a "Sidechain" button in the plugin window. You can click this button to turn the sidechain input off and on. If there's no sidechain input you can still toggle this button off and on for no reason. My suggestion is this:
If you click this button when it hasn't been assigned a sidechaining source, it will open up a list of your channels that can have the ability to facilitate sends. Click the channel and done, sidechain created.
This is better and faster workflow than the current method for a few reasons:
When you want to create sidechaining you must have your destination already created, which is the given plugin. This means that when you create the plugin you need to have sidechaining for, you won't have to leave this window, or look elsewhere to find the appropriate channel, click the button, find your newly created plugin in usually the bottom of the list of all the plugins that accept sidechain inputs. This is, quite frankly, tedious and not very user friendly, and it slows down the workflow. This reverse method would greatly speed it up, especially since you might also be likely to have less channels than plugins that accept sidechain inputs. Obviously this varies from project to project and what plugins you use, but for me personally the list of plugins that accept sidechain input is probably 3-4 times bigger than my amount of channels.
My second suggestion is a bit more complicated, but another workflow improvement over the current one (neither of these two suggestions exclude the other, and I would argue the first one should at least be made regardless of this one).
Here's a screenshot of a typical console when I work on producing a track (click for bigger image):
Now imagine if I were to shift-click the + sign on the sends box, to add a sidechain destination, and the console changed to this (click for bigger image):
Basically you enter a "select destination" mode directly in the console, where all plugins that don't accept sidechain inputs are faded out. The ones that do are slightly highlighted and the ones that do, but already have a source assigned, are tinted red.
Clicking on any of the sidechain capable plugin slots in the console will create a destination to it (and exit this mode). This, again, beats scrolling down a huge list of plugins as is the current method.
Naturally while in this mode you'd be able to use the inserts scroll bar, if needed, as well as the scroll bar at the bottom of the console to scroll through it. The mouse scrollwheel should naturally also still work. Pressing Esc, play or right mouse button (perhaps others that would make sense?) should take you out of this mode.
I would also like to suggest two improvements when creating a sidechain source that will just speed up workflow:
If shift is held down when user input is sent to create the sidechain source (be it using the current method or the two suggested above), it will be created a 0 dB.
If ctrl is held down it will be created with pre-fader enabled.
These two can naturally be combined to create one at 0 dB with pre-fader enabled.