At the moment our Talk message fields support only plain text. We should explore making those fields support the same level of wikitext + syntax highlighting, which we have in the general editing screen, and show the same toolbar of wikitext buttons on top of the keyboard.
(We can show the toolbar only for certain types of users, if necessary.)

In principle this shouldn't be too difficult, except for one technical issue:
In the Talk message field, we support @-mentioning usernames, which turns the username into a special Span object. This would be mutually incompatible with wikitext highlighting.

One possible solution would be:
After completing the action of @-mentioning, instead of creating a Span object, just output the resulting wikitext that corresponds to the @-mention. This is actually what the DiscussionTools interface does (in the Source editor, not Visual editor).
So, instead of adding @Dbrant to the message, it would become @[[User:Dbrant|Dbrant]] , with the cursor placed at the end of the insertion, as before. And if we introduce syntax highlighting, the latter username link would become automatically highlighted.