It’s actually already like that, where skills are individually added to the allowed list. That’s how you can now re-cast armageddon and hurricane while shifted, but still not cast any of the other elemental skills. This wouldn’t be a hacky change.
They also don’t need to nerf the cast rates, because aiming for FCR would mean significantly decreasing your damage. Also, the shockwave skill is based on FCR, so that would weaken an already low build.