Like the title says, it would be extremely useful to be able to assign a single single hardware knob/fader/encoder to the last touched parameter. Rather than intently assigning a parameter, users would simple click the parameter in the GUI and then tweak it with the assigned hardware knob. This is useful for quickly recording parameter automation without previously mapping the parameter.
This would need a mappable action to release the last touched parameter, in case you are using a wearable wirless MIDI controller or something that's continuously sending MIDI data.
In addition to all that, the one knob feature would need to support relative encoders (which Studio One already does) as well as, ideally, soft takeover modes. For that I'd point Presonus in the direction of Ableton Live's "pickup" and "value scaling" modes, which work rather well.
The way Studio One works at the moment is more geared towards creating permanent mappings for each plugin. Speaking ofthe cuff here but I'll hazard a guess that most users don't bother to create such mappings and just use the mouse because it's quicker and less of a hassle. That's why the one-knob solution would be a useful middle ground.
This feature would work extremely well not only with regular hardware controls (knobs, faders, encoders, etc) but also with something like the Genki Wave wireless ring controller.