Think about the traditional lisp printer and pretty printer. It's just that in this case pretty might potentially mean that certain parentheses could be omitted if indentation could still make the meaning clear.
So you might potentially have a printer that wouldn't attempt to remove parentheses, and a pretty printer that would do so to an extent.
As for the reader, if parentheses were optional but still allowed in normal s-expressions, then you could probably get away with using the same reader for normal and indented input.
sounds like a lot of work, but more importantly I feel that the reader and printer would lack internal consistency. That is, it would be harder for programmers to wrap their head around the reader/printer concept.
