So I really like this idea. After thinking about it, I think I agree with @
cbwjm that these abilities should be used at will whenever specific conditions are met. I like the idea of these being an alternative to Brutal Critical, and I think the key is making the triggering conditions a bit more generous than Brutal Critical. Here's my version that I think I will be using moving forward:
Primal Tactics
At level 9, you have learned to incorporate devastating tactics into the way you fight. You may choose one of the Primal Tactics listed below. Each ability describes when and how it activates. If a Primal Tactics requires a Saving Throw, the DC is equal to 8 + your proficiency bonus + your Strength modifier. If you know more than one Primal Tactic, only one can be used per turn. Using a Primal Tactic counts as an attack for the purpose of preventing your rage from ending early.
You learn an additional Primal Tactic at level 13, and again at level 17.
Thunderous Blow
Your strikes are so mighty that the force can be felt across the battlefield and cause nearby opponents to stagger back. When you score a critical hit or reduce a hostile target to 0 hit points, you can use your reaction to unleash a Thunderous Shout. All hostile creatures within 10 feet of you must make a Strength saving throw. On a failure they take thunder damage equal to 1d12 + your rage damage and are knocked back 10 feet. On a success they take half damage and are not pushed back.
Terrifying Shout
You have learned to combine your attacks with a frightening howl, causing your enemies to momentarily lose the will to fight. When you score a critical hit or reduce a hostile creature to 0 hit points, you can use your reaction to unleash a Terrifying Shout. All hostile creatures within 20 feet that can see and hear you must make a Wisdom saving throw or be frightened of you until the end of their next turn.
War Cry
Your rage is contagious, and through your voice it drives your allies to become more deadly. When you enter your rage, or if you use an action to activate this ability while raging, you can unleash a War Cry. Until the start of your next turn, all allies who can hear you within 30 feet gain a bonus to their melee damage equal to your rage damage.
Rallying Cry
You have learned to unleash a primal call that drives your allies to push beyond their normal limits. When you first enter your rage, you and all allies who can hear you within 30 feet gain temporary hit points equal to 1d12 + your rage bonus.
Ground Stomp
As an action while you are raging, you can slam the ground so violently that it can knock your enemies to the ground. Each creature within 10 feet must make a Dexterity saving throw or be knocked prone.
