As it so happens, // is going to be APPLY now due to slash's intimate tie to function execution.
So :: may be a reasonable candidate here, but I feel it needs to be optional.
In terms of strange musings on the power of FENCE!, I had an idea that it might be used to cast INTEGER! to DECIMAL!
>> number: 1020
>> {. number}
== 1020.0
Though here is where we get into keyword territory, because . would be redefinable (and it's looking like it will often be defined as a "this" or "self", to go along with .member acting as member selection).
So you couldn't do .: &[block] and get {. 4} to act like {block! 4}, you'd get 4.0
I'm wary of building in things like this, but we do have them built in here and there.