Probably a hot take, but I'd like an extended JSON syntax where you can do this:

    foo = "something to use later",
    bar = { "a": 1.1, "b": 2.2 },
    baz = [ 1n, 999999999999n, ],
    /* comments! */
    {
        [foo]: `\
    multiline template string
    ${foo}`,
        ...bar, /* spread! */
        "baz": [ ...baz, 1234n ],
        "more": my_extension("something like !foo in YAML, but just JavaScript function syntax")
    }