The Gimmick for SMN is what? The ability to summon 'pets' and pets have special actions.
The Gimmick for BLM is what? The Hot/Cold system (I know thats a real stripped down version)
The Gimmick of RDM is what? Insta-cast, with some hybrid melee combat.
These three classes have 3 specific gimmicks that affect their playstyle outside of their actual skills. WHen you boil down what raw skills they have, theyre actually a lot more similar. They all have an AoE skill, a few damage dealers, a dot or two. So on so forth. its how these skills are used in a system that differentiate the classes. They all learn skills in a similar enough vein, but how they use them and when is different.
Now, what is the Gimmick of BLU? It is to learn monster skills. Unlike the other classes, the Gimmick for BLU is the means of how skills are acquired. Not the application of them. They may have some little 'minigame' system (resource management or something) involved, but at the end of the day, the goal of BLU is to LEARN exotic skills and then use those exotic skills as they are to accomplish a variety of tasks. This creates the inherent problem. It's not just raw numbers that matter. Its what those monster skills would do.
Bad breath, as an example, would have to be dumbed down A LOT to work in a party environment. It would be a vastly smaller cone of effect, have most to all its debuffs removed, and likely deal minor damage (if at all). It would need this treatment so it woudl fall in line with all the other caster AoE spells. Then you have to consider which skills get gutted or not. If you make to many BLU skills have utility (i.e, make it to flexible), it would become an almost must pick. To few, and you limit the Gimmick of the class, stripping it of its identity. You cant have a wide array of skills either, because that is also powerful. Having more skills than any other class either means the skills are vestigial, or if theyre all viable, youd end up with an overly flexible class. Again, that would make them must picks in a raid environment. This is a big blind spot in points like yours. You assume power is merely a numbers game. X skill does Y damage, therefore it is/isnt powerful. But its not just skill damage, its what skills do, what BLU would have access too, and how that access interacts with all other classes. For example, if BLU could get Trick Attack, that would skew raid balance hard. Double the up time damage windows without impacting the LB bar, on top of providing everything else it might (buffs, heals, CC, you name it.)
If you start limiting what skills BLU can use and do in a party environment, then you end up chipping away at its Gimmick. The class becomes empty. Its just a caster with a few fancy spells that it learned from monsters. It identity and concept get reduced in scope and people then start saying "SE, you sucked at implementing this! This isnt BLU. This is a travesty!" If they add an extra gimmick to compensate, then its no longer BLU, because the Monster learning aspect is just a more complicated way to get skills, and the gimmick then becomes the second thing tacked on.
This is the problem that having it be part of the DF system would end up contributing to. Thats why its hard to balance. They can either give you a game experience of being a Mage who learns magic from monsters, and has to go on some quest or another and must use all the skills he or she learned to win (making the emphasis on learning monster skills), but restrict it so that it cant participate in end game content, OR they gut it, dumb it down, and possibly add a second gimmick, turning it into a shell of what the BLU concept was all so you can take a reskinned thaumaturge to party content. I dont know about others, but I prefer the first.
Ok, so you pretty much have a reskinned Thamaturge. See above to understand why. Even more so, but youre creating a system that has soft restrictions too. So any skill that isnt useable in a dungeon makes it moot simply because youre defeating the point of the party system, and any skill that cant be used on a boss is also moot, for the same reason. But now you have to remember what skills can be used where. We have that to an extreme limited extent. CC is generally tied to one or two skills at most, and most bosses are invulnerable to it. Your system would require more skills to become moot in boss encounters, and dungeons at large.
Aslo again, consider the power of having a class with to much flexibility. Lastly, you are adding a gimmick ontop of a gimmick. The core identity of BLU is learning skills, not the application of them in rotations. Where RMD, SMN, and BLM all have systems that use their skills in different ways (so the focus is on application within their gimmicks - like summoning Bahamut in SMN's case), BLU does not have such a system inherently. Their set up is "I have a scenario, and Ill use my plethora of skills casted as is to overcome it." Any gimmick system on top of that would either be very basic or would take power from the "Learned Skills" system for the sake of balance, thus mitigating the identity of BLU and making it more of a generic caster that happens to learn skills from monsters rather than an NPC.