"Now why stop there? You could try and remove more parentheses by using a tab syntax, or introduce a few other irregularities, or both. There's a good chance that we could come up with something better than the currently widely accepted surface syntax."

The constraint is that the grammar of the resulting syntax has to be capable of expressing everything expressable by S-expressions, has to be unambiguous, and it has to be pretty-printable back into a canonical form. The last point itself is pretty hard to do, but if you actually look at the grammars of a lot of programming languages, very few of them aren't ambiguous!