The "plans" are for South WCML, South ECML, GEML to all be (the perpetually stalled) IEP using shorter interurban units (double up in peak).
Yes, that's why I was thinking 444s (or possibly an end-doored Electrostar variant) would be ideal - nothing too complicated, a known quantity, quick to build and compatible with many of the modern new units. I'll be amazed if IEP ever climbs out of stall and goes ahead in its current form.