I wrote:
> A better proposal than using different category codes is 
> programming macros with optional arguments obeying 
> a convention that makes a choice of delimiters available to users.  
> Any macro package replacing \@ifnextchar[ by some 
> \ifx\let@token\opt@arg@delim could be run this way. 
> 
> If no LaTeXY will adopt this, I will call it LüTeX :-)

I had thought of posting such a "feature request" earlier, 
refrained from it for not having found a good idea how 
to implement it. If I now found it so simple, I must have 
overlooked a problem ...

This would mean that so many \foo in the LaTeX kernel 
reading their optional argument by \@foo[#1] must be replaced.

At least I have now thought how such macros would have to 
look like instead (sketch, \SetOptArgDels chooses delimiters, 
\foo is an example command with one optional argument):

    \def\SetOptArgDels#1#2{%
        \let\opt@arg@del@l#1%
        \def\read@pass@opt@arg##1#1##2#2{##1{##2}}}
    \def\foo{%
        \@ifnextchar\opt@arg@del@l
            {\read@pass@opt@arg\@foo}%
            {\@foo{DEFAULT}}}
    \def\@foo#1...

-- where one had \def\@foo[#1]... so far.

Maybe this already is in xparse? Sorry, I cannot afford more right now ...

Fortunately, 

1. there would be no need to change preamble-only commands, 

2. as I realize at reading Heiko's comment, the "ambiguity" only 
arises with macros whose *last* (e.g., only) arguments are optional.

Cheers, 

    Uwe.