OSX's Keyboard prefs lets you define custom menu shortcuts for any program. Sadly there's no global interface for stuff like "hold this to zoom" so you still have to deal with whatever the program's developer thought sounded good.

In the Gnome 2.x days, you could (in GTK apps) assign a custom keystroke to any item in any pull-down menu by hovering over the item and then pressing the keystroke you wanted to assign. It was the most simple, intuitive way to customize app keyboard shortcuts I've ever seen, and I still miss it.

My OS has that (KDE) sort of. Non-KDE apps don't use it because it's OS-dependent.

You'd need all platforms to have this and then a multiplatform abstraction layer to exist before it became useful... and by useful I mean: you can start rewriting every application.

Isn't this kind of what Apple is trying to do with the TouchBar?

Shame that the TouchBar is hot garbage.