The Empire had various designs up their sleeves, but not all of them were picked. Look at the TIE Advanced x1. What became of it? It got put aside, and the TIE Interceptor was born instead based on the Advanced x1 research. Further development was put into the x1, but it was slow. Eventuelly it resulted in the TIE Avenger, but still the Empire didn't want a shielded, hyperspace capable ship. They wanted big ships, and refused to realize that they really needed TIE's to be shielded. When they finally gave in to their own ideals, it was too late. So, based on this mindsett, I have no doubt that if you as the Empire prevent the rebels from aquiring the X-Wing design, it will most likely be scrapped, or morfed into something else.