Fusion could introduce many new features that wouldn’t require a new post-processor. For example, a new type of toolpath generation will still generate a toolpath, which is most likely handled by the existing post-processor and almost certainly using existing G-code commands.
Avid’s post-processor doesn’t support some things. For example, see this thread on simultaneous 4-axis machining. You might have to make post-processor changes to get a specific ATC setup fully working. There are certainly other examples I don’t know about.
If there is a feature you need in Mach4, you can write custom Lua scripts and run them in a modified screenset or map them to M commands and include them in your G-code. If someone needs to change how G-code itself is interpreted, then Mach4 is probably not the right controller for them, as it is closed source.
I’ve not heard of anyone tweaking ESS except to change how signals are mapped. If you’re that far down, again, you’re probably interested in using something other than Avid’s standard control setup.
All that said, if you have an example of a new feature that requires a change to one of these elements, some of us on the forum might be interested.