Red is continuing their investment in a kind of string interpolation:
FEAT: String interpolation by hiiamboris · Pull Request #5085 · red/red · GitHub
They are defaulting to parentheses instead of $, and escaping parentheses with backslash (\on the inside, curiously\)
It's worth thinking about what a good default is, though ultimately I think it should be customizable (as with historical REWORD). I had been considering plain parentheses for the same reasons at first, but leaned to $ just because of how popular it is in other languages for interpolation.
My concept is different because it is not a hardcoded preprocessor behavior, but making binding information available on strings as a map of words to values. See "Rebol and Scopes: Well, why not?" for thoughts on that distinction and the challenges it presents.