Smalltalk had a completely extensible syntax

Interesting finding at this page

You will be interested to learn that the very first version of Smalltalk (Smalltalk-72) had a completely extensible syntax (in fact the writing of a class also automatically supplied the grammar). This worked very well, except ... that too much freedom here leads to a Tower of Babel as far as other users are concerned. This has also been the experience with the few other really good extensible languages (like Ned Iron's IMP).

Extreme extensibility was removed in the next major design of Smalltalk (Smalltalk-76) in favor of a syntax that could read by anyone, regardless of how many classes had been defined ... i.e., getting stronger meanings turned out to be more important in the end than making language structures fit the task.