In the 2017 wikitext editor, the MediaWiki:Edittools aren't available. These Edittools, or their project specific versions like w:de:MediaWiki:Onlyifediting.js (in combination with w:de:MediaWiki:Edittools) or w:en:MediaWiki:Edittools, offer several elements that are not available in the 2017 wikitext editor, like the entity , the tags <nowiki></nowiki> (also combined to <code><nowiki></nowiki></code>), <includeonly></includeonly>, <noinclude></noinclude>, <onlyinclude></onlyinclude>, or matching character combinations not available in the "often used" section of the character menue (in de-wikipedia e. g. “” and ‘’) and much more. These featueres should be made availabe in the 2017 wikitext editor, either by using Edittools, or on a different way. Beyond that, also elements like <br />, {{}} (for parser function and variables as they can not be added by the Template wizard) and {{{}}} (for parameters) would be helpful.
Topic on 2017 wikitext editor/Feedback
The "Often used" section of the special character menu is under local control, so the local community can add whatever they want.
Generally speaking, VisualEditor's wikitext mode is "tuned" for article editing, so the team is hesitant to include elements that should not be used in typical Wikipedia articles. If you frequently create templates, then you may be more satisfied with an editing environment that is optimized for that kind of work. (The "old" wikitext editor is still available for one-time edits, even with the beta feature enabled; just change the end of the editing URL to say &action=submit instead of &action=edit).
(I see that you have found T164956 about the line breaks.)
The "Often used" section is the same in the VisualEditor and in the 2017 wikitext editor. To include tags there would probably not be such a good idea, as VisualEditor is intended for WYSIWYG-editing. It would be good, to have a menue especially for the 2017 wikitext editor or a 2017-wikitext-editor-specific extended version of the "Often used" section to place the HTML-entity or tags like <nowiki></nowiki> or <code><nowiki></nowiki></code> there. Thank you for telling me the possibility to change editor via URL-modification. In the de-wikipedia it only works, when one has disabled the VisualEditor during it's Beta-Phase in ones settings (only then, one get's the old wikitext editor, if one replaces "veaction=editsource" or "veaction=edit" in the URL by "action=edit").