Putting larger engines on the existing ATR is not as simple as it may sound. Due to the increase in power, controllability in the event of an engine failure becomes a problem. This would require a redesign of the rudder, and as a result the large aerodynamic forces would most likely require hydraulic flight controls. And as we know one thing leads to another, so a simple re-engine can suddenly take a lot more time, not to mention money, and at the end of the day you just end up with a stretched 72. It would be much better to start with a clean sheet, and design a new aircraft using the latest technology and materials.