For example {{SomeTemplate|some-param}} is represented in the parse tree as

<template><title>SomeTemplate</title><part><name index=\"1\"/><value>some-param</value></part></template>

Instead, it could be something like

<template><markup>{{</markup><title>SomeTemplate</title><part><markup>|</markup><name index=\"1\"/><value>some-param</value></part><markup>}}</markup></template>

so that loading the parse tree into a DOM model and getting the textContent of the root element would return the original wikitext.

This is a breaking change so we'd need a version parameter and support for both versions, which seems straightforward.

The parsetree preserves whitespaces and the markup is represented by the tags, it is already possible to remap the wikitext from the parsetree, not that complex, but also not easy as with textContent.

Some hints:

• <template> => {{ ... }} possible with lineStart= attribute
• <tplarg> => {{{ ... }}}
  • both contains <title> and optional <part> with <name>, <equals> and <value> each, for each part a pipe is needed.
• <ext> => < ... > with <name>, <attr>, <inner> and <close>, when close is missing it is represented as short tag with />

The wikitext in <inner> is not expanded, same reason as T4700.

For a deep dive into templates you would need the inclusion parsetree, that needs T51353.