{{TOC}} is a bit ofrisk regarding templates that currently exit and do this already.

{{#TOC}} would work I guess, although some of the options listed there seem like overkill / too many options, the following sound useful though. Implementation ala [[File:]] sounds great, so that the order won't matter, nor are parameter names required and the values can be easily localized for content language.

• left/right (unnamed parameter, default left, inserts a class, styled via shared.css)
• maxlevel=[1-6] (dont show headings higher than this level)
• class='' (adds additinonal classes to the default 'toc' class, such as visualClear, or fancy-toc)

A replacement for Compact TOC might be nice too.

In T293513: Deprecate and remove ParserOutput::setTOCHTML() I am moving the HTML generation for the ToC to ParserOutput::getText(). It might be easy to add a hook here to allow an extension to change the style of the rendered HTML. Would that work?

@Krinkle's suggestion could probably be implemented to extend the information which goes into the JSON output. It probably wouldn't be hard to allow that to be extensible, so that any args given to {{#toc|foo|bar}} would be visible in ParserOutput::getSections() and thus could be used by an extension to customize the ToC.

Note for the new Vector __TOC__ will not be supported given the table of contents will be always place din the sidebar, however, we are very interested in replacing use cases such as https://en.wikipedia.org/wiki/Template:Compact%20TOC

(

For example if an editor did:
{{#TOC:horizontal=yes}}

We'd be interested in having access to { horizontal: true } in the Skin where we render the table of contents and have special handling for it, so the direction sounds good from my perspective.

Change 884102 had a related patch set uploaded (by Jdlrobson; author: C. Scott Ananian):

[mediawiki/core@master] Provide ability to customize the TOC title, id, and class

https://gerrit.wikimedia.org/r/884102

I have to make a decision about default case for T204370, but we might want to make this {{#toc}} (aka lowercase) instead of {{#TOC}}. The recommendation from Unicode is to make the word case-sensitive, not case-insensitive, since the rules for case folding are hard to localize well.