It has been mentioned in some places[1][2] that the JSON syntax might be a problem for typical (non-technical) users, so I suggest creating a plugin for editing the content of the <templatedata> tags.
[1] https://commons.wikimedia.org/wiki/Template_talk:Information#Adding_TemplateData_information
[2] https://pt.wikipedia.org/wiki/WP:Café_dos_programadores#Mais_uma_novidade:_TemplateData
Version: unspecified
Severity: enhancement
See Also:
https://bugzilla.wikimedia.org/show_bug.cgi?id=51734