Problem: In Visual Editor, a template within another template cannot be edited properly. Not only does the user get wikitext (which should not happen in Visual Editor), but it is also confined to a tiny field in the template editor window. Whenever the main part of a template has to be inside another template for technical reasons, this makes the template editor and TemplateData for that template useless.
Proposed solution: Nested templates should be detected and every single template should be editable using the template editor of Visual Editor.
Who would benefit: Users of Visual Editor.
More comments: I remember first discussing this at Wikimania 2017. Unfortunately, there seems to have been little progress on the matter.
Phabricator tickets: phab:T52355 (at least part of it)