The XC ones could be greatly improved by a decent refurb, you could fit in more seats by changing them to ones with a thinner back, and by increasing train lengths (e.g 4 car to 5 car, 5 car and HST to 8 car) with the avanti 221s.And these short comings are not related to the underlying design concept, a full strip out of the interior and refurb, then either reform into longer units (if thats possible?) with some new units to take up the slack or cascade to less busy routes would address most of the issues. I realise that the cramped feel is due to loading gauge constraints introduced by the need to tilt, and you cant do anything about that, but if the other issues were addressed this would seem less significant.
Other problems like bad fuel economy, too many disabled toilets (part of the structure) etc. Cant be fixed.