Probably the best thing we could do here is to mirror the options available in similar tools, such as jq: https://stedolan.github.io/jq/manual/#Invokingjq

The relevant options here would be:

    --indent
    --tab
    --compact-output
    --sort-keys

The default indent in jq is 2, which I tend to prefer these days, but maybe 4 is still appropriate given PEP 8:

    $ echo '[{}, {"a": "b"}, 2, 3, 4]' | jq
    [
      {},
      {
        "a": "b"
      },
      2,
      3,
      4
    ]


This is how jq interprets --compact-output:

    $ echo '[{}, {"a": "b"}, 2, 3, 4]' | jq --compact-output
    [{},{"a":"b"},2,3,4]


I do not think that it's worth having the command-line tool cater to people that want to indent in other ways (e.g. using a string that isn't all spaces or a single tab).