Yeah, you got it there. Synergies don't have to apply just to static cooldowns and raid buffs. It's just stacking multipliers based on their respective timing windows, after all. The optimal numerical option emerges naturally almost irregardless of what you want to do. And that is basically how 90% of the synergies in this game operate right now, even the ones I'm about to talk about.
I am referring more to interactions like Celestial Opposition extending Lucid Dreaming or Deployment Tactics and the Fairy, where individual parts of your own kit can come together to give you more, and you usually have to consider them if you want their full benefits. You can build whole classes based on manipulating portions of your own kit to achieve a stronger effect than the sum of their parts. Some classes that I feel do a better job at achieving this currently would be BRD/MCH/SMN for DPS, SCH for Healers, and DRK for tanks. A party example of this would be Bard and Dragoon synergizing with one another because of Crit Stacking on Bard specifically. Regardless, tiny examples of this do exist in all of them currently, just not to such a degree that it's meaningful.
It's also possible to add more interplay via anti-synergies as well. SCH has Dissipation and DRK used to have Dark Dance and still has DA-Dark Passenger, both of which don't work well with Blood Price. SMN achieves this rather accidentally via Bahamut and the Aetherflow lockouts, and MCH's interactions with skillspeed also are an example of this, the difference is the results come off as punishing and clunky, disrupting flow, but encouraging more forethought, which can be a good thing. In the case of DRK I would consider my cooldown sequencing a lot more because of those interactions. In the case of SMN? I'd rather focus more on Aetherflow/Bahamut itself than worry about timing Rouse properly. And I think that speaks to how I view their respective roles and the mechanics therein.
To give another analogy, compare it to Magic the Gathering:
- In general, should your class feel more like a value deck, an engine, or a combo deck? In other words, how much synergy do you want?
- All DPS would be aggro decks because of course they are. However, do you want your dps class to feel like a by the numbers Burn or Mill Deck? Or do you want them to feel like Affinity, or Merfolk?
- Do you want your healers & tanks to feel more like control, midrange, or tempo? All deal with pressure thrown at them in different ways but ultimately care about synergy differently because of it. Control would probably map best onto old style MMOs, where your focus is entirely spent on dealing with the encounter using your best value tools and proper use of those tools will secure you an inevitable victory. WoW would be closer to tempo, focusing on efficiently responding to an encounter with more limited tools that may or may not work well together but ultimately can't keep up with the encounter forever (whether due to enrage mechanics or simply not enough statistical power), Midrange is analogous to what we currently have in FFXIV, every tank/healer can respond to an encounter but also contributes a significant amount of DPS to help close it out.