One of my favorite parts of the AGE system used in the Dragon Age RPG is the stunt system. When making an ability check in AGE, you roll 3d6 to get your result. If you succeed and at least 2 of the 3 dice show the same value, then you generate stunt points that can be used to add special effects to your action. I think that this system offers a much more interesting design and play space than the critical hit rules in most games because stunts happen more often (44% of rolls get doubles vs. 5% of d20 rolls being a 20) and they offer a menu of options besides just increasing the damage output.
That said, I think stunts could be a lot better integrated into the game as a whole. There is a lot of design space in the stunt mechanics and some Dragon Age RPG designs also work against the stunt mechanics. One of the big things that feels like a missed opportunity in AGE is when an active character doesn’t roll. For example, the High Dragon in Dragon Age RPG Set 3 has a fire breath attack that allows it to breath either a wide arc or a narrow stream of flame that deals damage to everything in the area. Targets can make a Dexterity (Acrobatics) check to take half the normal damage. While that design is widely used for area attacks in RPGs, I feel that it is problematic in AGE because the dragon doesn’t make any roll on the attack and so has no opportunity to use stunts.
As a quick conversion, you could use a house rule that replaces the fixed TN in attacks like the dragon’s breath with a roll that has a bonus equal to the attack’s TN – 10. For example, the High Dragon would make an attack roll with a +8 bonus.
I feel like a set of defenses similar to those used in Dungeons & Dragons 4E could be a better solution, but it would involve deeper changes to the AGE system. Using a defense could also eliminate a lot of places in the Dragon AGE RPG where both an attacker and the defender make rolls, such as offensive spells, so that only active characters roll.