For example, to learn and use the Warfare source skill "Overpower", your character would have to have a minimum of 5 points invested in the Warfare ability. Perhaps at 8, you could now learn and slot two Warfare source skills into your spell deck, etc. These are all arbitrary thresholds, the specific numbers themselves would have to be determined by balancing of course.
This would be similar to how D:OS1 had Adept and Master skills locked from use until you reached a higher mastery in the respective school.
I like that idea. Right now Source skills are absolutely not worth it because of the 3 Memory cost. Yes, maybe later on in the game, we'll have Memory and skill slots coming out of our wazoos, but for right now, in Act 1, on the island of Fort Joy, we just don't have enough Memory to afford 3-Memory Source skills.
Consider the boss fight at the end. That's the kind of thing you want and need your most powerful stuff for, but to get the memory space needed, you need to cripple your loadout some other way. And that fight is hard so crippling yourself then comes at just the wrong time.
If Larian still wants power gating for Source skills, a minimum ability requirement might work. The downside would be for Hybrid builds and poly-school casters, both of which would find it harder to be able to use Source skills compared to single-school specialists.
