This will be a new editor, not a modification to the existing wikitext editor, because we want to avoid suddenly disrupting editors and breaking all existing gadgets.
MVP (for initial release as a Beta Feature) is described in more detail at T141149: Provide a Beta Feature of a modern wikitext editor integrated into the visual editor.
- Section editing
- Desktop, tablet and mobile-suitable (dynamic/responsive)
- Syntax highlighting
- Code folding
A rough mockup is here. To see the wikitext editor, click the brackets icon in the top-right corner.
A rough demo video as of mid-May 2016 is https://www.youtube.com/watch?v=jgd2ZHOZGBE.
You can see it in action at https://visualeditor-test.wmflabs.org/wiki/Wiki_markup?veaction=editsource
See also: mediawiki.org page