After 4 pages I didn't see a JA suggestion for BLM, here's a few:
Something loosely based on say... Tomahawk that would nullify magic shield (and related abilities) [see: Magic Shield - FFXIclopedia ]for a short while (possibly removing Colibri reflect, etc.) Certainly it is Black Mage appropriate, akin to a situational Dispel.
Either that, or it could be a JA like Elemental Seal that affects the next elemental spell cast to break through special resistances (and not just general ones), could possibly have a residual effect like AMII/DNC steps. It certainly wouldn't be game-breaking, and it would be a fairly unique ability.
Certainly anything that affects SC/MB would be nice, perhaps post MB the mob's magic resistance is obliterated temporarily, could use SC/MB to land magical effects on mobs that otherwise wouldn't land like party triggered Elemental Seal that's triggred when a blm lands the MB
You could even call it Elemental Grave or Stamp or Mark or what have you.
Well, now I'm just rambling.
Anyway, it's not so hard if you try to create tools to keep BLMs slightly more versatile without nullifying party diversity. I think BLMs deserve a little boost, just because the job has become stale, vanilla, crusty, etc.