At the (online) language summit yesterday, our erstwhile BDFL responded to a suggestion to use '?' in annotations. He said that many ideas arose over time to us the question mark for various things, but he felt none so far were compelling enough to exclude some future better use.
I actually find the LOOK of ':var' attractive enough and could easily imagine wanting to use it. E.g. maybe someday we'll get a nice deferred evaluation system; I think that could look very nice as a way to indicate such a deferred term.
This purpose of just not typing a name twice in a dict display isn't very important to me. And I just showed a silly hack to do basically the same thing with a function that constructs a dictionary. If we add new syntax, it should do something important rather than just be syntax sugar... Yes, f-strings are arguably a counter argument.