Normally, there would be three facets of context for complexity and therefore decision-making (or, the portions of cognitive load typically called "decision-making"):
- the toolkit,
- content-interaction, and
- party-interaction.
For brevity, as it is the densest area, we'll mostly ignore the last for now, pretending it synergizes with, or at least does not conflict with, the toolkit and content-interaction.
To be clear, obviously not all are required to make for what I have so far called a "decision". There need only be enough complexity for there to be multiple relevant frames of reference (next weave, next GCD, next string, next burst, etc.) with competing answers to what would be best and for all but the best players to be able to definitely estimate a most productive frame of reference.
In XIV, this complexity has been typically drawn from content-interaction, since party-interaction has been made basically foolproof and automatic (even prior to simplification and consolidation, due simply to either pre-fight configuration as per set comps or the best choice being too far from the next best to be overshot in any situation). Even content-interaction, though, has diminished over the years, with fights behaving increasingly like target dummies, though not always for that great a loss to complexity; they had already rarely ever met the breaking points within a kit precisely enough to offer choice, prefering instead to offer waves of differing actions with a more obviously correct choice, as per interrupting distinctly AoE phases in an otherwise ST fight.)
Of course, all is bottlenecked by the opportunities available to a given kit (a duo of targets means nothing if you have no DoT [combo] and your AoE has no relevant advantage whatsoever until 3+ targets), so I'll try to cover that, too, specifically but generically.
Yes, party interaction and such can also reduce complexity. If you would have done something differently in order to survive but an ally could just throw onto you and otherwise wasted shield anyways, they may get some greater access to complexity in that context, but you will have less. Your would-be choice becomes theirs unless your choice is the more efficient and theirs the backup that is nonetheless better spent on you than someone else. Etc., etc.
As such, let's just assume that if the example below takes a cross-role action like protecting itself from unavoidable AoE damage or briefly gathering or tanking adds, that the party has no competing option of greater efficiency.
Some general factors contributing to complexity that may, if sufficiently balanced, contribute to decision-making. For now, axe those low-hanging fruit of universal undermechanics that would be beyond XIV.
Stagger Systems (Damage can function also as mitigation and/or --granularly or at certain dynamic thresholds-- damage amps, offering less fixed competing tempos for optimal use.)
Resource Managment (Usually requires granular resource effect, such as having less throughput as one has less Stamina. Requires a toolkit balanced sufficiently to allow for burst vs. sustained action. For this to result in complexity, there must be a desync (between the burst/sustained phases enticed by resource management and what the kit would oblige on its own) balanced sufficiently for the best choice to be situational. May or may not include effects for/from %HP that ["artificially"] raise the value of defensive play, especially just before one's own throughput bursts.
Behavioral Manipulation (Mobs have, by species, a different behavioral script by which to determine target selection and their own "optimal" action, usually in accordance with base-threat-time-script-multipliers. Mobs may, as an action, change scripts or their threat coefficients may themselves be influenced by existing total threat, unit-specific threat, %HP, lead threat's %HP, etc.)
Physics-based Effects (Being able to knock enemies up, back, down, etc., based on action weights, positioning, and mob context. Often granular. Slightly redundant with stagger systems in terms of complexity, but often complementary to overall gameplay feel.)
Elemental Effects (Like physical effects, but using instead manipulable spectrums like Heat, Shock, Saturation, Mass. Complementary with behavioral manipulation and physics-based effects, but with some diminished returns regarding complexity.)
- Factors Affecting Uptime (Boss jumps. Movement requirements. Boss-radial AoEs of greater than melee range. Proximity based AoEs. Almost never offers any meaningful choice in itself but can allow for it if there are sufficient complementary factors.)
- Factors Affecting Target Count (Add spawns. Almost never offers any meaningful choice in itself but can allow for it if there are sufficient complementary factors.)
- Factors Affecting Relative Damage-per-Action (See Ravana Red/Green/Blue phases. Sometimes redundant, sometimes usefully competing with party interaction.)
- Randomization (of the selection, target selection, frequency, and timings of mechanics).
- "Tethers" / "Baits" (being able to bait mechanics or otherwise force their target selection -- relevant only if they spawn away from the otherwise best positioning for their ideal recipients, at an opportunity cost in movement that neither obliges nor forbids their getting said tethers).
- DPS/Healing/Mitigation checks (relevant to complexity only when competing with otherwise optimal actions, such as STing during AoE opportunities to burn down a dangerous mob or not using CDs or bankables during a certain raid bursts in order to deal with a DPS check soon after).
- Unavoidable damage that would situationally force non-free personal defensives. (Technically not in XIV anymore, but we've had them, or near enough to them, for some jobs at some points. Most relevant if they affect the flow of burst phases, but that's pretty much only been a quarter of a real thing in XIV, maybe per Stormblood DRK's TBN if its Blood spenders had carried gameplay-affecting additional effects.)
Okay, so we basically are allowed just DPS checks, boss jumps, AoEs that'd force at-cost movement
(so, not a thing for decision making it itself for XIV Physical Ranged jobs), add spawns, tethers, randomization, and maybe painful-but-not-too-painful-AoEs if the kit has a way of addressing those mechanics.
Alright, that's our ceiling. Now, we need a kit that can interact with them. I'm just going to use a DPS as my example kit because those are the most numerous jobs and have the highest portion of independent complexity.
Where that kit gives a clear, straightforward fit to a given feature of a fight, we have only engagement through execution and there is little cognitive load. Where it fits less cleanly, usually via some degree of inertia (an AoE also having a higher degree of guage generation, which is more bankable, or AoE bankables being more efficient relative to their ST equivalents than AoE GCDs are to their equivalents, etc., so a next-GCD decision can affect a capacity for an upcoming mechanic), there's greater cognitive load (approaching a "decision").
- 4 enemies? No conflicting DPS check therein? No decisions, just dance.
- 6 GCDs against 2 adds, between ST dps checks wherein the boss reappears and would nearly wipe the party unless his cast-HP is burned down (and where one's 2-step AoE combo falls slightly short of their 3-step ST combo in ppgcd but has higher gauge generation)? Nearer to a decision.
One could (A) just AoE spam so the boss arrives on an AoE opener (and can spend bankable oGCDs as AoE or ST, more flexibly), (B) downclock 1 GCD and prep the AoE finisher for the check, or (C) AoE twice and then prep an ST finisher for the boss's brief dps check. The best answer therein will depend on one's party (their gear, attention, bankables), any prior mistakes/irregularities (overspending bankables, simultneous mechanics, etc.), and the severity of the dps check (including whether it's all-or-nothing).
<Ran out of time. Releasing now in hopes for a bit more detail on what degree of concreteness you want. Will be back later.>