Easy rule to implement, yes, however it means they are balanced differently. Let's say Aggressors are Core for Ultramarines but not Imperial Fists, Aggressors should cost more inherently for Ultramarines than Imperial Fists, while the Plasma toting Primaris could be core for Dark Angles so they should cost more.
The reason for the base cost difference is due to being able TOO be affected by rerolls.
The issue is, I dont see GW doing that which means it is likely an all or nothing. If they surprise me and do it that way and take the time to increase points by 3-5ppm then it would be interesting and fun.
Regardless of all of that, what I want is the Forge World book... i want to know what is happening to my Chinorks!