Mastery should most definitely feel like an integrated part of the spec. It doesnt necessarily need to be massively complicated, but something like "X% chance to Y".
The old MM mastery up to WoD was kinda like that. X% chance to fire an extra arrow. You can then make this extra ability do extra stuff that makes the spec feel more alive.
At the very least each mastery should give some unique utility if nothing else. Again with the MM mastery, it isn't particularly exciting as a DPS increase since it just increases damage by a flat%, but at least it gives you more range, allowing some interesting playstyles when or if you build it up high enough.