3) Open-endedness. The other big failing I see in attempts at Blue Mage classes is attempting to provide a specific list of abilities that can be stolen. This is how Final Fantasy does it, but it doesn't really have a choice because it's a videogame. This point kind of overlaps with number 2, but the consequences are different. If a Blue Mage can look up what they can get, then not only is there no mystery to the adventure, but it also outright precludes a whole swath of character concepts. If the only thing you can steal from a dragon is that breath weapon, because the breath weapon is specifically marked as stealable but nothing else is, then you cannot create a character whose goal is to become as dragon-like as possible by hunting dragons near-exclusively. I've never done this personally, but there's nothing stopping a Vorpal Knight from taking the claws from one dragon, the wings from another, the breath weapon from a third, a fourth for the tail slap, then the natural armor, so on and so forth. They would pay dearly in opportunity cost to do it, because each of those dragons could have been something else, but they COULD do it. There's a sort of mix-your-own element to a good Blue Mage system that almost drops a miniature classless system inside any other system.