* assign a bill to `a` so that `a` will evaluate to the value of the name `foo` any time that `a` is evaluated, in the scope of that evaluation

   a = $foo

* as above, but always plus one

   a = $foo + 1

* make `a` a bill that evaluates to the value of the name `foo` at the time that `a` is evaluated, in that scope, plus the value of `bar` **at the time and in the scope of the assignment to `a`**

   a = $foo + bar



You can do all that programmatically. No need for magic operators.
Check this module as a proof of concept: https://github.com/jbvsmo/funcbuilder

foo = FuncBuilder()
a = foo + 1
# Then evaluate a with a function call

For it to do exactly what you want, you just need override the evaluation function (Funcbuilder.__call__) to load names from current frame locals or something.
BTW, I don't recommend using that on real code because it is extremely experimental (unless you're a hipster... In this case, carry on).