Skill/Spell Mastery

[:1]I think creating an upgradable skill/spell mechanic would be an excellent way of creating diversity in builds...

Along the lines of Tales of Symphonia, where if you use a skill a certain amount of times under the right conditions, you can suddenly "learn" a new and more powerful variant of that skill.

Obviously it wouldn't be as simple as "use skill xx amount of times"... but something like this could be an excellent way to make lower tier skills a viable endgame option.

Thoughts?