I was just looking into using macroexp.el and found some features that I
felt were lacking.  These were: 1. accepting multiple forms in
`macroexp-if' and `macroexp-let*', 2. flattening of `progn's in
`macroexp-progn' and `macroexp-unprogn', 3. getting rid of branches in
`macroexp-if' in case the TEST is constant (and consequently a way to
tell whether a constant form is nil or non-nil).  I've went through the
rest of macroexp.el and haven't found anything else that stood out,
though I might change my mind as I keep using it.

I've attached a patch for possible implementations of these, though this
is mostly so that I put this out there as a suggestion; considering how
many things depend on macroexp.el there might be good reasons why these
features are not worth the effort and possible bugs that come about due
to these changes.