Elements

[TheDustyOne]

While I enjoy elemental systems, it's a system that has to be designed around with from the start of development. As it is currently; BLM would not be able to fight against any fire or wind mobs and would be extremely broken against ice and water mobs, RDM would suffer from very bad mana alignment against fire and water mobs, as well as lightning and ice mobs while fishing for procs, WHM would be as useless against water and earth mobs as BLM is against fire and wind. There's also elemental abilities that people often forget about in most kits, like NIN having Lightning, Wind, and Water, and MNK having Fire, Earth, and Wind (although they exist only as buffs that don't directly change elemental affinity).

Between all the casters (including the upcoming Pictomancer), WHM, the 2 melees that prominently have elements in their kit, that's 7 jobs that need to be reworked from the ground up and 10 years worth of content to redesign. You then have to ask how much of a bonus job A should have over other jobs in fights because of the elemental differences; should it be fair for job A to have a 10% bonus over job B because the boss is weak to Fire and job B has Ice? And if you think it's unfair for there to be large discrepancies, then you have to introduce all elements into any elemental job for parities sake, which makes the system very homogenous and heavily bloats job kits. That's not to say that it's impossible to have missing elements in kits, but it's very difficult to balance around.

All this to say; this game wasn't designed for it and you're better off asking for a new game that incorporates such systems. There's too many bosses, mobs, and jobs to redesign from scratch to make it work and could negatively affect the enjoyment people have with those jobs now.