The Warrior's Job: Protecting allies

[Tintalvar]

I suggested a variation on Hârn's engage rules to my players:
Whenever you enter a hex adjacent to an enemy (their engagement zone) with the intend of passing through, you have to beat that enemy in a Dex (Initiative) roll or your movement ends.
If the winner of this roll has not taken a major action this turn, they are allowed to immediately do so (mostly to attack).

You could even go the whole nine yards and require that roll every time you enter an enemy's engagement zone (like Hârn does).

[Saisei]

I'm wary to start telling my players about engaging and disengaging etc. I don't want them to get 4e flashbacks (The tactical unimmersive combat is why we have left 4e for now). That said, I do want to have some kind of penalty for just up and abandoning a fight with an opponent. I might do as mentioned and say you need to take a Minor action to safely move away, guarding against any attacks.

Or also have that as long as you act against the opponent (i.e. by attacking him) you can freely move away after that. The idea being you wonk the guy on the head and while he's thinking about how much that hurts, you leg it.

[extrakun]

Old time wargames have Zone of Control. This is similar to Rules of Engagement that some posters have already mentioned. Of course, the problem with ZoC is that you need a tactical map.

For combat which does not use tactical map, here is an idea from Spirit of the Century/ Fate 3. A party member can defend an ally who is nearby by substituting his action as a defend action. So in DA, here's two more actions

Defend Ally: You are there when an enemy strike your designated ally in combat. The target of the enemy will switch to you instead; however, you have to roll your Defense, which is 3d6 + Dexterity because of the complex maneuvering required. However, as this is a defensive action, you still gain a +2 bonus.

Interrupt Enemy: You attempt to strike an enemy who is about to attack your ally. Make an attack as usual and match it against the enemy's attack roll (not Defense). If you roll higher, you cause damage and disrupts the enemy's attack. If not, your ally will be targeted as normal.

Of course, both actions only work for melee attacks. This make bows even more dangerous. (After all, archers do naturally go after mages).

Both actions delay the character's turn to the end of the round; or they can pass that their turn so both actions persist to the next round, if their Initiative is lower.

[Zapp]

If the winner of this roll has not taken a major action this turn, they are allowed to immediately do so (mostly to attack).

Being able to make an attack first is a big thing in a gritty deadly system like Hârn.

In a game like Dragon Age, where a single is blow is very unlikely to kill or even slow you down, I suggest the antagonist if he wins that roll gets to make a free attack, rather than merely making his regular attack now instead of later.

[Balgin Stondraeg]

Defend Ally: You are there when an enemy strike your designated ally in combat. The target of the enemy will switch to you instead; however, you have to roll your Defense, which is 3d6 + Dexterity because of the complex maneuvering required. However, as this is a defensive action, you still gain a +2 bonus.

Hmm,. A simpler version of this would be to allow the Defensive Stance stunt to be "given" to a nearby ally (possibly costing an extra stunt point).

Interrupt Enemy: You attempt to strike an enemy who is about to attack your ally. Make an attack as usual and match it against the enemy's attack roll (not Defense). If you roll higher, you cause damage and disrupts the enemy's attack. If not, your ally will be targeted as normal.

Hmm, that stunt that lets you make an extra attack could allow you to lash out at an ally's nearby foe (to weaken it and maybe cause it to give you some attention too).

I like your alternatives. I just feel the need to comment on how similar things are already possible with the existing rules (to a less specialised extent).