@dpk makes a good point; / is far more common in prose than * or _. Furthermore, there are cases where a forward slash is surrounded by spaces, such as in catalog formats (e.g. Gone with the Wind / by Margaret Mitchell ), so we probably can’t count on spaces on either side of the / for formulating the syntax rules for starting/ending a span of alternative voice.
There’s also the problem of a single forward slash potentially conflicting with the content block syntax which uses a / at the start of a new line to identify a file path to transclude in the document. We might be able to work around this by identifying a delimiter forward slash somewhere on the line.
It wouldn’t be out of place to use a double character pair in CommonMark (strong emphasis, for example). Using a double forward slash seems to be most natural here. Here’s what the examples above look like using a double forward slash to mark up alternative voice/mood.
The //Felis silvestris catus// is cute.
The term //prose content// is defined above.
There is a certain //je ne sais quoi// in the air.
Raymond tried to sleep.
//The ship sailed away on Thursday//, he dreamt. //The ship had many people aboard, including a beautiful princess called Carey. He watched her, day-in, day-out, hoping she would notice him, but she never did.//
//Finally one night he picked up the courage to speak with her—//
Raymond woke with a start as the fire alarm rang out.
It’s not quite as elegant as using a single forward slash, but it’s still a presentable way of marking up alternative voice/mood.