Just a heads up, this is just about having a discussion. It's not anyone's job to have solutions and it's not necessary for anyone try shutdown discussion or get defensive. The forums are here to trade ideas and discuss them!
As a game that started to help ease new players into the MMO genre, I think that this MMO need to grow with its players. We've seen the introduction of new skills and mechanics in 3.0 to add some further complexity into the game, but there's only so much more you can add before it gets too difficult or complicated. Since combat plays a significant part of this game, it's important to keep it interesting. The combat isn't necessarily bad - it still plays better than basically every other MMO in the market, but there is still something lacklustre about it. We don't have options to change our experiences, each class has the same way of playing (there are no points to improve certain spells or change the way they are used besides a handful of cases - e.g Astrologian).
I think we should consider elemental strengths/weaknesses somewhere in the future to enhance the combat in this game. Obviously that would require redesigning how some of the classes play, and it is definitely something that occurs (this game wouldn't exist without the intention of reimagining/redesigning aspects of the game).
Black Mage would obviously need to be redesigned, but I think that would be for the better. I actually applause SE for the current Black Mage, because they managed to subtly and somewhat incorporate the original design of Thaumaturge (being an umbral/astral caster) and the Final Fantasy Black Mage archetype we all love and know. The current rotational system for Black Mage works for now, but how many more layers can we add before it's not player friendly? I think SE has the creativity to redesign the class to fit a combat system with elemental strengths/weaknesses. What this would require is less vertical spell progression (e.g tier 1: Astral/umbral, tier 2: Enochian, tier 3: ???) but more horizontal progression (more situation spells and 'spell trees'). While the combat in 1.23 was very simplistic, I think it did a better job at encapsulating what a Black Mage is, especially when we look at what Black Mage has been in Final Fantasy. Perhaps if Black Mage gets redesigned, we can take the umbral/astral identity and create a new class/job from it (there is plenty of information on it with the original Thaumaturge pre-version 1.20).
I understand that in 2.0 Yoshi-P wanted to move away from the old system of building up your power over the course of a fight, so maybe that needs to be taken into account. I'm sure there are still ways something like this could be done without going against that principle, though.
I think elemental attributes also grants the opportunity to improve other classes and diversify the experience. If we take Monk for example - which originally was an elemental focused melee dps, it's now (as in, since 2.0) kind of similar to Dragoon with it's huge reliance on positionals. Perhaps the need for positionals can be changed/removed/traded for situational elemental rotations to bring back the old Monk identity that we once had, and give them more situational roles.
Elemental attributes also means new summon opportunities, and new ways in which unaspected/non-elemental abilities (as seen with Lancer, Arcanist, and Archer) can be utilised.
Reintroducing elemental attributes may only be one part of the changes this game may need. We may need other changes that the community feels is important. Maybe something that needs to be reevaluated is the notion of DPS/Tank/Healer, since the classes/jobs we have are much more than that niche role. There is a lot of room for expansion, maybe we need to reconsider what constitutes a party? Should it be 5 members to include an optional 'Support/Utility/Dps' slot?
This game still has so much potential. We've seen the uniqueness and creativity this game has once had and still has in many other aspects, so I have no doubt we can expect some interesting changes in the future.