I made definition commands that can be used to produce commands and
environments with optional arguments.
Because of TeX's lack of being a general purpose language, these
definition commands have some restrictions: The conditional parsing is
non-deterministic (= slow), picking down some general look-ahead portion
(right now the whole paragraph), so one cannot enclose such a command in
braces "{}" not containing the look-ahead portion (right now, a \par) after
the command.
So the stuff is experimental, and I am not sure it is worth wrestling
with TeX, getting around these problems, but I thought the specs being of
interest for a discussion of what might be possible in the LaTeX3 project.

The command \define used to define commands with optional commands works
as follows:
\define\newtheorem?[#1#2[#3]{def1}%
?[#1[#2]#3{def2}%
#1#2#3{def3}
The question mark "?" before the parameter text means that the parameter
text is optional, and the conditional character to look for is the next
following the "?", a "[" in this case. If this condition comes out true,
LaTeX style, the definition following it is executed, otherwise the test
passes to the next row. (If you want non-conditional parameter text
starting with a "?", you can start it with a exclamation mark, so by this,
all conditional/non-conditional parameters are possible).
Apart from being an implemented definition command style, this is also a
convenient way of indicating conditional arguments.

The environments conditional arguments works along the same principle:
A definition
\new{Environment}{bar}?*#1*#2{[1: $\alpha(#1)$]}{[1: $\beta(#2)$]}%
#1#2{[2: $\alpha(#1)$]}{[2: $\beta(#2)$]}%
??#1?#2{[1: $\gamma(#1)$]}{[1: $\delta(#2)$]}%
#1#2{[2: $\gamma(#1)$]}{[2: $\delta(#2)$]}
would produce an environment named "bar", with hooks and optional commands
at both the begin and end groups.
The "?" on the first line indicates that it is conditional, with a test
character "*". If the test comes out true, something corresponding to
[1: $\alpha(#1)$]\begingroup[1: $\beta(#2)$]
plus keeping track of environment variables, will be executed; otherwise,
it skips to the next line.
As this next line is non-conditional, the line following it
??#1?#2{[1: $\gamma(#1)$]}{[1: $\delta(#2)$]}%
determines the endgroup. This is also conditional, with test character "?".
If this test is true, something like
[1: $\gamma(#1)$]\endgroup[1: $\delta(#2)$]
plus keeping track of environment variables, will be executed; otherwise,
it skips to the next line.

In addition, one needs not indicate the name in the environment endgroup,
corresponding to "\end{}". I have also abstracted so that one might use
different styles of environments end and begin groups (a constructor that
creates the constructor that creates envrionments). This might be useful
for making environments with syntax closer to say HTML, or something,
depending on what you want. When abstracting the environments, it became
clear that the "\begin{foo}", and "\end{foo}" stuff is best treated as a
shell, or syntaxctic soup, calling the real commands (which are called
\foo/begin and \foo/end).
-- Also, the idea of creating modules, with internal command names of the form
"foo/.../bar"
seems to be highly workable.

But it is difficult getting around those shortcomings of TeX, working in
this generality, and so it may not be worthwhile spending a lot of effort
on that.

Hans Aberg