Right now all events report after-the-fact. It should be possible to suppress keys, keeping them from being processed by applications down the line. This allows users to create global hotkeys without affecting the focused application, for example.

This is possible in Windows, but would require moving the hotkey processing back into the hook.

Linux seems to be trickier. Maybe relying on X, like https://github.com/alols/xcape ?

Finally, extreme care must be taken to not introduce input lag. Because all hotkeys will be processed in the main thread, blocking the event they analyze, it would be too easy to add precious milliseconds to every key press, which is not acceptable.