You're pushing it too far. Having one logical step per line is better for readability, clarity, and debugging-friendliness.

> Programming language features should be orthogonal as much as possible.

True, and that's why string building and string formatting should be two things.

By the way, I am not sure about how would this PEP should handle something like

    a = 'A1'
    b = f'{a}'
    a = 'A2'
    s = f'{a}' + b

Said otherwise, is it possible to create at run-time an f-string?