And what about those extensions that add only 64bit data types in places where 32bits where allowed? Will it be hard to add?
At least in the past, there have been a bunch of places that assume that every data type is the same size (i.e. you can stuff ints/unsigneds/floats in the same bits without losing anything). That would need to change. Plus, we currently don't have any 64-bit data types in our shader compiler and assembly backends. These are not really hard problems to solve, but there's enough that it's non-trivial work and probably a bunch of tedium.