In short: Protect users' hearing by allowing them to switch on a customisable cap for master volume output.
Due to various issues (e.g. software bugs, feedback, unexpected plugin effects, unbalanced presets, etc.), it's always possible for a DAW to suddenly output audio at a much higher volume than expected. This can be very damaging to people's hearing & nerves, especially if using headphones.
It would be great if S1's preferences included an option to turn on/off a user-configurable threshold (e.g. 0 dB, +1 dB, +10 dB; whatever the user wants), anything above which (on the master output only) will be automatically muted or capped at that level.
The meters should still accurately display the true level, preferably with some sort of indicator to show when the cap has kicked in.
A great example of this functionality already implemented is Cerberus Audio's Ice9 Automute. It does exactly what's needed, but this sort of protection should be out-of-box, rather than needing a third-party VST.