This applies more generally; no need for any weird hacks. Any 'new' plugin could just opt in to a different syntax; we can just look up until the first ':'; we just need to define a new interface for a new syntax.
I don't think that this provides a good user experience. 1) There are existing endpoints that want nestable endpoints, so either a) They don't change, somewhat defeating the purpose of having a new syntax (or cluttering the endpoint namespace with less than useful endpoints). b) They change incompatibility, defeating the purpose of trying to maintain backwards compatability. 2) As user, I need to learn which endpoints support the new syntax, thus potentially needing to know both methods of quoting and switch between them as appropriate. There are a couple of possible ways around this, without requiring a weird hack. - I wonder how many endpoints strings have ever been written whose value starts with any of `[` `(` or `{`? I suspect that the number might in fact be 0. In which case, although the change is technically incompatible, in practice it wouldn't be. - Alternatively, we could deprecate an unquoted [, (, { at the beginning of a value, and then after a suitable deprecation period (perhaps additionally a release where it is just an error), we could repurpose one of them to act as quoting (leaving the other two for future extensiblity).