Weapon Actions Appendix C: The Master of Battle and Other Subclasses
One possible concern with the Weapon Actions subsystem is that it treads too much on the toes of the Battle Master subclass detailed in the 2014 Player's Handbook (pp. 73—74), as some of the benefits granted by weapon actions overlap, in part or in whole, with the benefits granted by certain 'manoeuvres' from this subclass. Similar issues may arise with the Ranger's Hunter subclass (SRD 5.1, pp.37—38), or with other similarly tactical martial subclasses introduced by other official or third party materials.
It should go without saying that in games where no player is interested in taking any of these 'problem' subclasses, this potential conflict is a moot point. I would even go so far as to say that even in cases where a player takes one of these subclasses, the additional benefits of the subclass features relative to weapon actions (typically including increased damage, more frequent availability, etc.) allow these two features to compliment each other quite nicely —the player may choose to either double-dip on a single penalty (such as knocking a creature prone or disarming them), using both weapon actions and subclass features to gain additional opportunities to inflict this condition, or they may choose complimentary subclass features and weapon actions to provide more tactical options during combat.
That said, it seems desirable to offer these tactical martial subclasses some way to interact with, and benefit from, the Weapon Actions subsystem. I propose doing so by granting these subclasses additional features at the level the subclass is first gained. The following are three plausible example features for three such subclasses— the Fighter Battle Master, the Ranger Hunter, and the Bard College of Swords. Note that of these three subclasses, only the Ranger Hunter is included in the SRD—mechanical details for the other two subclasses in full can be found on the DND Community Wiki (see Battle Master and College of Swords). The below features carry the usual playtest disclaimer.
Weapon Mastery6
Level 3 Battle Master
Also at 3rd level when you choose this archetype, your martial prowess allows you to wield weapons with greater finesse. You can expend 1 superiority die to use a weapon action of a weapon you are wielding and proficient with, in addition to your one use of that action per rest. If you do so, you can roll that superiority die and add the number rolled to the damage roll of any attack made as part of that weapon action.
Hunter's Weapons7
Level 3 Hunter
Also at 3rd level when you choose this archetype, you can channel your connection to nature into your martial prowess. You can expend a spell slot of 1st level or higher to use a weapon action of a weapon you are wielding and proficient with, in addition to your one use of that action per rest.
Weapon Flourishes
Level 3 College of Swords
Also at 3rd level when you choose this archetype, you can take advantage of various weapons in your martial displays. You can expend one use of your Bardic Inspiration to use a weapon action of a weapon you are wielding and proficient with, in addition to your one use of that action per rest. If you do so, roll that Bardic Inspiration die and add the number rolled to the damage roll of any attack made as part of that weapon action.