I'd been pondering the "only unquote once, as late as possible" rule, since that's effectively what URIs do -- there's one quoting scheme, and effectively each layer only treats non-quoted characters as special and passes quoted ones on to the next layer down. This only works because the layers don't conflict over their special characters, but I wonder if you could combine this with Tclish nestable quotes to get something useful.

It also occurs to me that you mostly don't pass shell commands to programs as a single quoted string, but as the tail of another command, which is kind of reminiscent of the way the URI syntax is defined to allow an entire URI to be used with no additional quoting as a query string.