I've played a bit the idea that barthard sketched.  I don't have all the details worked out, but I believe this is what will happen:

With

  parser = argparse.ArgumentParser()
  parser.add_argument('-w')
  parser.add_argument('-x', nargs='+')
  parser.add_argument('y')
  parser.add_argument('z', nargs='*')

some possible parses are

    '-w 1 -x 2 3 4 5',     # w:1, x:[2,3,4], y:5, z:[] -
                           # fail +
    '-w 1 2 -x 3 4 5',     # w:1, y:2, x:[3 4 5], z:[] +

    '-w 1 -x 2 3',         # w:1, x:[2], y:3, z:[]     -
                           # fail +
    '-x 1 2 -w 3 4 5 6',   # w:3, x:[1,2], y:4, z:[5,6] +
                           # w:3, x:[1], y:2, z:[4,5,6] -

    '-x 1 2 3 4 -w 5 6 7', # w:5, x:[1,2,3,4], y:5, z:[7] +
                           # w:5, x:[1,2,3], y:4, z:[6,7] -

    '1 2 3 -x 4 5 -w 6',   # w:6, x:[4,5], y:1, z:[2,3] +
      
'+' lines are those currently produced
'-' lines are ones that would be produced by these ideas

'-w 1 -x 2 3 4 5' is the protypical problem case.  The current parser allocates all [2,3,4,5] to -x, leaving none for y, thus failing.  So desired solution is to give 5 to y, leaving -x with the rest.

'-x 1 2 -w 3 4 5 6' is a potentially ambiguous case.  The current parser lets -x grab [1,2]; y then gets 4, and z the remainder.  But the alternative is to give 2 to y, leaving -x with just [1].

In this case 
arg_strings_pattern = 'OAAOAAAA'

replacing the Os with the option flags: '-xAA-wAAAA'

I match this with a refined version of bethard's regex:

    pat1='((?:-wA)|(?:-xA+)|(?:-wA-xA+)|(?:-xA+-wA))'
    pat = _re.compile('%s?(?P<y>A)%s?(?P<z>A*)%s?'%(pat1,pat1,pat1))

groups (without the Nones) and groupdict are

    ['-xA', 'A', '-wA', 'AAA']
    {'z': 'AAA', 'y': 'A'}

So this does effectively give y the 2nd argument, leaving -x with just the 1st.

The current parser effectively groups the arguments as

    ['-xAA, '-wA', 'A', 'AA']

In the real world, generating and apply a global pattern like this could get complicated.  For example there are long option names ('--water'), and combined argument strings ('-x1', '-x=1').