These are some of my thoughts as to what I would like to do to PLT. 
Most of these ideas require a serious amount of coding and therefore
may never get implemented.

1) Macro commands.  Currently 'Plot Vertical' automatically configures
a series of plot windows.  In principle, this command just runs
a series of more basic commands to set up the windows.

This should allow NxM windows to be easily configured.  For example,
'Plot 3x1' would create a vertical stack of 3 plots containing the
first 3 active groups and 'Plot 1x3' would create a horizontal row
of 3 plots.

2) Total rewrite of COD.  COD is currently modelled upon Forth but
with a real stack.  I think the HP calculator language would be a
better model which would allow for algebraic expressions.  Since this
is a superset of what COD currently does, I think it may be possible
to do this in a way that existing COD files would continue to work.
Would also like to be able to enter a COD function on the command
line.  This would allow for a class of models that are slightly more
complicated then the builtin functions, but not complicated enough
to justify writing a "program" in a COD file.  (I know some users
are scared of the concept of writing a COD file.)

3) User friendly 2D fitting.  I don't think there are many useful
2D model components.  Take the simple case of a polynomial.  In the
1D case, the Powerlaw allows users to easily construct a polynomial
in X of any degree.  One could easily do the same for Y and would
need to include the cross terms, X*Y, X*Y^2, etc.  Thus the general
powerlaw would be

         Norm * X^N * Y^M

where Norm, N and M are fit parameters.  Of course, this does not
include a rotation, where there is just one index but along an axis
not parallel to either X or Y.  What would this function be called?
Would the typical user realize that get something that varies as Y^2
then you would freeze N=0, M=2?  This appears to be more of a "trick"
than an obvious use of the function.  Would a typical user ever need
this function?  I really think an algrebraic COD function entered
on the command line provides the easiest most general solution.

4) Plotting a 2D model as an image.  This is tricky in that PGPLOT
assumes the image is stored in an array, whereas in this case it
is a function.  Some thought will be required.