I think part of the reason these discussions tend to end up with people constantly talking past each other is that actual players are more complex than broad groupings properly-capture.
All of these people actually exist in FFXIV:
The player who actually gets angry if you suggest changing their "freestyle" rotation that ignores major Job mechanics and potency logic, because they're "having fun" or "roleplaying".
The player who egotistically sneers at anyone who's not optimizing their rotation in even trivial ways. Usually they communicate in a Pidgin English composed primarily of Twitch emotes and Discord emojis.
The player who's pretty damn good at their Job, but doesn't really mind if other people aren't, and just kind of chills and plays the game well with their own dedicated group of like-minded players.
The player who desperately wants to earn the social accolades that they perceive as coming from parsing fabled colours like orange and pink, but just doesn't have the inherent chops to do it, and becomes agitated and frustrated about every last optimization as a result.
They player who's not really here to parse; they just like the challenge of playing their Job well. Their parse is a side-effect of doing that, and a measurement tool, but they don't consider it a colour-coded superiority contest.
The player who really just wants to see and clear all the content, no matter how hard it is. Playing their rotation correctly is a side-effect of wanting to be able to clear difficult content, but it's not their primary reason for being here, so they're not that bothered about how easy, hard, depthy, or shallow a Job is, but instead focus on how tactically-useful it is.
The player who's just chillin'. They participate a little bit in all content. They try to read their tooltips and follow the correct rotation, but if it's too much work, practice, stress, or requires spreadsheets or timelines, they're probably not bothering. They like it when a Job feels "a little detailed", but not "too oppressive".
The player who's just kinda here for the fun fantasy world. They truly don't care if their White Mage spends most of the time Glaring, because they're not really here for the combat depth, but rather, seeing the Story, housing, etc.
...and on, and on, and on.
It's maybe not even a "sliding scale", but more like a "kaleidoscope", and basically each "tier" of complexity will end up turning on, turning off, or not bothering either way, different segments of this relatively-broad and complex population.
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What I'm trying to say is that you can certainly crank up the fabled "ceiling" on Summoner — but how much?
I believe that you'll have someone disappointed at every new "increment" of complexity — and in both directions. Because it's ultimately a very personal and subjective determination what "a good ceiling" actually is.
For example:
There are players who would be genuinely-happy if Ruby Rite and Slipstream were turned into instant casts, and Crimson Cyclone was turned into a ranged attack.
There are players who would be genuinely-happy if Summoner just had a few more failure-points, an "actual" filler phase (ie, casting), and maybe at least one "real" Gauge to manage.
There are players who would still "eh" away from Summoner unless its design reached full-sweat theoretical Black Mage tiers of complexity, like ShB SMN on cocaine.
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...I think that every time someone says, "I just want this Job to have more skill expression" (or, conversely, "I just want this Job to be less difficult to play"), it's always unconsciously-coloured by that player's very personal and subjective limits.
For example:
Let's say that Summoner damage was increased to be higher than Black Mage.
However, literally all of Summoner's actions are now 4.0s casts, and each Gem phase has a strict 16 second timer that begins when a Summon cast finishes.
That would certainly make Summoner much more challenging to play... but it would probably be abandoned by very nearly everyone. Because, while it would take "skill" to execute that rotation successfully, even dedicated players would probably find it too oppressive to be worth the trouble.
However, there would probably be, say, 10 die-hard players around the globe who would passionately enjoy the challenge of figuring out how to play "Slow Summoner" in actual content.
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I have a concern that arguments about "skill expression" and "skill ceiling" really don't have an objective standard, and inevitably end up being something like:
...And which Job belongs in which category slides around depending on who the beholder is.
- Easy for me to do even without practice → "Braindead"
- Easy for me to do with a bit of practice → Poor skill ceiling
- Hard, but I can do it with practice/planning → Good skill ceiling
- Hard, but I can't do it, even with practice → "Overtuned"
I say this because, if "skill expression" was really an objective, universal quality, Jobs like BLM and MNK seem like they would have higher play rates in higher-end content. Instead, they tend to be minority picks, even amongst seasoned players, which leads me to think that everyone has their own threshold about what kind of "skill ceiling" and "skill floor" and "skill expression", and so on, that they actually want.
To be clear, I'm not intending to mock any of those "skill" criteria, nor saying that you're wrong for considering and valuing them — I'm only trying to caution that it's probably not so simple to "Goldilocks" a given Job to the point that you actually want it to be at... and that if you manage to get it that way, it will probably end up being a personal, rather than universal, fit.