Here's what I thought of, and this may(?) be more original than other systems. At least, I haven't seen a script of this yet.
Rather than directly leveling the skills themselves, what I was thinking was leveling up Elemental Proficiency. As characters use skills of a certain element, their proficiency with that element increases and they gain various benefits related to that element. Possible benefits, that I thought of, include:
- Increased damage
- Reduced SP costs
- Increased scope (make some single target skills able to multi target)
- More Skills (either by learning/forgetting skills, or by permission flags (skill_can_use?))
- Increased resistance to that element (this is probably out of the scope of the lesson though)
Probably some other things I haven't thought of yet, but that's a good amount to work with at this point.
Another possible feature of the system might be a Polarity system. Simply put if two elements are in opposition, like say, Fire and Ice, then gaining proficiency in Fire would reduce proficiency in Ice, and vice versa. This would promote specialization in one or two elements.
What does everyone else think?