Yeah, we do have plans to add more instruments, including a 'drums' section in the future.
Many things in the game will be expanded on 'later on' - but we cannot give a timetable as to when they will be added (some of these features will only come out after the release and not before).

The "Early Access road map" sounds interesting though - since we do have some features that we consider 'must have' before the official release of the game - we'll look into it :)