Here are a few ideas I had:

Dunharrow Swordsman (ally, tactics) Cost: 3

Will: 1

Attack: 2

Defense: 1

HP: 2

Oathbreaker. Warrior.

Defiant 2.

Response: If Dunharrow Swordsman is destroyed by an enemy attack, deal 2 damage to that enemy, then place Dunharrow Swordsman at the bottom of your deck.

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Defiant keyword: add up the defiant number on all characters you control. At the end of each combat phase, you must discard defiant cards from play until your ready heroes' willpower is equal or higher than your total defiant value. For each card discarded in this way, raise your threat by its defiant value.

Commanding Presence (attachment, unique, Leadership) cost: 3

Attach to a Dunedain or Noble hero.

Response: After you play an ally from your hand, attached hero gets +1 willpower until the end of the round.

Blood of Kings (attachment, Spirit) cost: 1

Attach to a Dunedain or Gondor hero.

While ready, attached hero gets +1 willpower.

Pressed into Service (event, Tactics) cost: 0

Action: Reduce the cost of each ally played from your hand this round by 1. Each of those allies gains defiant 1.

Army of the Dead will be a Boon card in a RotK Saga Expansion.

it's a permanent attachment, game text:

Choose 1 of the following: destroy all enemies engaged with a player or destroy all enemies in the staging area." Remove Army of the Dead from the game."

or "Add Army of the Dead to the victory pile to divide x damage over enemies in play where X is the willpower of the bearer of this attachment."

naah, too crazy.

As long as they don't make it Outlands 2.0

Defiant is a cool keyword concept. Did you make it up or does it already exist?

I always imagined Oathbreakers as neutrals with text "Cannot be declared as a defender or take damage."

I always imagined Oathbreakers as neutrals with text "Cannot be declared as a defender or take damage."

But then maybe having them removed from play after a certain period of time? Otherwise even a 1 attack non-unique ally would be crazy if it can't be destroyed.

The fading mechanic from MtG or something very similar would work well on oath breakers (cones into play with x counters, 1 is removed each turn and when none remain the card is discarded)

And how it's gonna be crazy? It's not like you can play 10 oathbreakers in the turn 1. Being unable to defend means you'll need someone to defend, and if you won't have that someone in time - you'll have a dead hero before oathbreakers can even attack.

If the "or take damage" was removed, it might be less crazy. It would be fully feasible to see one played each turn, in addition to other cards (like a defender or readying effect), giving you +3 attack that can only be removed by very rare treachery effects.

But, hypothetical discussion and, with Stone of Erech having been released since the saga expansions started, I don't know how much of that story segment we will get to see.

Assigning archery damage to oathbreakers? What?

It's simple - you don't make outbreakers insanely powerful.