Bear in mind, these things may change by the time SB comes out, but if any of them stay the same it is worth taking a look at these strange little quirks of the game which aren't at all intuitive.
1. DoTs and Debuffs
So DoTs are a big thing in this game, all of the Jobs put up at least one DoT (or should) and the physical DoTs in particular have a strange property. While the weapon you use to inflict the DoT might have a particular type (Blunt, Slashing, Piercing) the DoT you actually apply is not that same affinity. That is to say, if you are playing DRG, and you apply Disembowel, your Phlebotomize and Chaos thrust DoT won't tick any higher. This is because while your initial hit is considered a Piercing attack, the DoT on the enemy is considered generic physical damage, and hence unaffected by any particular debuff.
This is something that commonly comes up with new players, and I asked this myself when playing WAR: Does Fracture get boosted in damage by Storm's Eye? It's a slashing attack, so you'd think so, but it has no effect whatsoever.
To compound this, effects like MCH Hypercharge and Trick Attack do affect the DoT potency. So despite the fact that Disembowel is functionally identical to Hypercharge for piercing damage it does not have the same effect.
Now, I will freely admit that the synergistic debuffs are incredibly powerful, and heavily influence the meta in mid to high end raiding. DRG would be even more appealing in a comp if it also buffed BRD DoTs and MCH Lead Shot.
Even so, there should be some sort of consistency here.
2. Suiton and TA
This is sort of a minor thing, but the fact that you will only really know that Suiton is used for TA is by mousing over the effect after you've already casted it is a bit strange. On console it's not exactly easy to mouse over a status effect. Add the words 'or Suiton' to the end of the TA description to make things a bit clearer.
3. Foe's and Boss Arenas
A QoL thing for Bards, but please change Foe's so that it only begins its effect when you attack an enemy a la Hypercharge. As Arenas change in shape to accommodate different boss shapes and mechanics, we have issues like Zurvan and A9 where precasting Foe's becomes an issue where you're guaranteed to aggro the boss. Beyond that, accidentally having Foe's on while straying near an enemy you'd rather not aggro ( such as a dungeon, 24 man, so on) can cause issues.
These are just a few thing my friends and I have considered over playing this game for the past year and a bit. While some might not be relevant come Stormblood (Debuff synergy) things like the Suiton description and Foe's aggro are going to be consistent little things for the duration of the game's lifespan if they don't receive (hopefully) small fixes.