Adding new markup is complicated*, I'd rather prefer a KISS approach, not least because of backwards compatibility.

The only possible targets in PM is <a name="anchor-name"> so adding a parser for a magic href #toc or #notoc would already do, i.e.

<a href="#toc" name="anchor-name">

A new markup <toc> should have a reasonable default behaviour

• nesting all H tags, unless excluded by linebreak
• adding all <a name=""> tags under the last H level

Any "filtering" features should be discussed after implementing and testing the first phase.

In a side note: There is a tendency to mangle the concept of "keywords" into this discussion.

IMHO that's mostly unrelated, first things first.

FWIW PM has already a keyword nodelet, but lacks a search engine for it...

*) actually I can't even locate the corresponding code, I'm volunteering to implement it if possible.