I disagree: this is not unlike including the schema in the JSON itself. This should be handled by the apps themselves, since they would have to know what the keys mean regardless.

If you do want the interchange format to be the one deserializing into specific runtime data structures, use YAML. YAML's tag syntax allows you to run arbitrary code inside YAML, which can be used for what you want.

I'm not talking about something arbitrarily extensible. Just a few more common data types -- primitive-like values that frequently need to be passed around.

I'm not talking about compound values like x+y or lat/lon.

This would probably best exist as a well-known wrapper around JSON itself.