I have seen games which have the mechanic where as you deal damage or take damage, a certain meter goes up, and once it is maxed, you are able to use a super move.

In theory, such a mechanic could be applied to Source skills, so that they are unavailable at the start of combat until the participants - both enemy and allied alike - deal and take enough damage to power up their Source skills

But those games are almost entirely JRPG's, and I don't remember a lot of western RPG's using mechanics like that. I have a hard time believing that Larian would warm up to that idea, especially since it conflicts with the other methods of gaining Source from the now-removed pools, to Soul Jars, to Source Vampirism.