Now after this excure in layers ...

... coming back to xparse: to a large extent that implentation provided one
possible incarnation of a document-level user interface modeled after current
2e, in other words it was meant to provide a user interface in which you can
define user commands that look like current 2e commands and offer the same
kind of possibilities (like a * or optional arguments present by [...])

It should therefore probably be better named xparse-2e or something like
that. After all, it is 

  a) quite possible to envsion more than one document level interface being
     available in parallel

  b) a 2e-like interface, while needed to allow supporting 2e documents is not
     necessarily the best interface for the future

  c) depending on requirements and environment on that level interfaces might
     make use of additional tools to manage the input stream

if we accept that the current xparse is really xparse-2e (whether we call it
that or not) then one consequence from this for me is that xparse-2e should
offer the possibility to define commands with a syntax that fits current 2e.

I would expect that once we have a clearer picture of how to do the separation
between layer -1 and layer 0 all this needs rewriting anyway

I also hope (and expect) that once we are clear on how to write specifications
for layer 0 properly, that other interfaces for layer -1 will be written, both
because more than one might be needed and because we need some trials to
settle on what we want to promote as that standard layer -1 for latex3

frank