Making a citation is one of the major VE workflows that still doesn't have a keyboard shortcut (EDIT: <ref works, but my sense is that we don't want to advertise that to new users). One candidate is meta + option + f, which Microsoft Word has used for adding a footnote. However, this conflicts with Google Chrome's shortcut for "search the web".
Comments from T59452:
My first choice is ctrl-alt-r, which unfortunately is in use as a shortcut to open Special:RecentChanges, and command-R is in use by the browsers to reload pages.
I'll personally use this enough that I'll be able to get used to just about anything. Maybe ctrl-alt-1, if you want this to work across multiple languages? Adding citations has "number one" importance to many Wikipedia editors.
Let's avoid symbols not available on non-English keyboards for a start, like backslash :p
Ctrl-Alt must never be used for any keyboard shortcuts if you want the editor to work on non-US keyboard layouts, since it's equivalent to AltGr on Windows, and AltGr is often used to input special characters. For example, on the Polish keyboard layout, AltGr+A produces the 'ą' character (and there are eight more similar shortcuts for 'ć', 'ę', 'ł', 'ń', 'ó', 'ś', 'ź', and 'ż'). If you take over the shortcut, these characters become impossible to type.
@matmarex, do the existing keyboard shortcuts work with a Polish keyboard on a Mac? Control-alt is used by MediaWiki software for several browser/OS combinations in https://en.wikipedia.org/wiki/Wikipedia:Keyboard_shortcuts#Modifier_keys
They would probably mostly work for MediaWiki, since you still can use the combination, as long as you're not typing text in a text input. They wouldn't work for VisualEditor, since you're always typing text when using it.
But I actually don't see Ctrl+Alt listed for any browser on that page? (Note that MediaWiki doesn't control the combination, only the last key in it; the modifier keys are browser-dependent.)
The logic, incidentally, being that we already have the "add a link" shortcut as meta-k, and references are (to the end user) fairly conceptually similar to links, in that they're both "add a pointer to more information somewhere".
Plus, you know, it wasn't taken.