@TrippyLighting, I agree on the use of referenced components and assemblies. You should be able to make changes from the top level. Believe it or not, this is high on our list, but it's a bit of a difficult problem to solve. I'll resist the temptation to explain why...
One point here, though, that I would like to make: There are two very different operations that are possible. Internally, we call these "Component Features" and "Assembly Features". The differences are a bit subtle, but important, and only show up if you have multiple instances of a component in your top-level assembly.
A "Component Feature" refers to a feature that is owned by the assembly, but which affects all instances of a lower-level component. Imagine that you have an array of instances of a simple block component:

If you sketch on the face of one of these, and draw a circle:

Then, do an Extrude Cut, a Component Feature will change the definition of the shared geometry by all of the instances of the component:

This is what you can do, today, in Fusion, if your components are all local to the design. If they are external references, you cannot. This is one project - to implement this for external components.
But, what I believe @Anonymous is asking for is what we call an assembly feature. In an assembly feature, the extrude only affects the instance it interacts with. Fusion does not do this today, but this is kind of what it would look like if it did:

This very accurately mimics a common manufacturing workflow. You buy instances of components, assemble them, then modify them in their assembly configuration. A good example is assembling a couple of standard plates, then drilling a hole in them to bolt them together. The holes are not necessarily in the same place on each plate, so they cannot be in the component itself. This is a separate project for us.
Hope that clarifies the differences for everyone.
Jeff
Jeff Strater
Engineering Director