I would remove the Opportunist talent and add it's mechanic to the single handed and two handed abilities. Because of their small length Dagger should not get an attack of opportunity (AOO).

At level 1 the damage of AOO should be reduced. An AOO is a panic moment: "Shit, he is moving, I wasn't expecting that." It's a quick jab not a full swing.

For higher levels of the single handed and two handed abilities they could add the chance to slow or cripple the passing foe.