I've taken to wearing bone-conductors waaaaay to much, and I'm impressed with their flexibility for stuff like this. They keep my ears open, but when I need to hear the headset more clearly (like if I'm in a crowded area and taking a call) I can plug my ear with my finger and that both improves the audio quality of the bone conductor (it creates a speaker cabinet in your ear for a much fuller sound) and blocks out the outside noise. If you need full headphone-quality you put in ear-plugs. And they're pretty small and discrete.

They're not perfect, but the fact that I can move smoothly from "I need my ears open but still want to hear my headset" to "I need to block out sound and hear my headset perfectly clearly" with just a finger or a pair of earplugs is great.

Stick a shotgun mic (that's the term for a mic with tight directional cone, right? Not an audio guy) on the side and this would be really cool.