Quickly going through these:
- Yes, this is the main gripe for many players.
- No, this leads to certain jobs being under/overpowered dependant on party makeup.
- Just jobs in general.
- No. You can make a job that is faster, but don't make it universal.
To expand on some points, starting with the 2 minute meta. By getting rid of it completely, you stop the need to have every job be a burst job that bursts every 2 minutes, it also means you do not need to keep designing things that fit into that 2 minute time window, like high potency attacks. Instead, you can then focus on making jobs more unique in how they burst, or not. More sustained DPS so their burst window doesn't register much on a DPS curve, high burst jobs where the sustained damage is low, but they burst really high, jobs than can have a more flexible burst window, longer windows, shorter windows etc. Jobs can really come into their own here. The main problem that might arise is fight pacing. With more downtime, a fight might favour one job over another, just like the situation with Pictomancer and FRU.
We also do not need to get rid of raid buffs entirely, Bard's can maintain a damage buff with 100% uptime, Dancers can maintain Standard Finish. Raid buffs that can be kept up 100% of the time are fine, as they do not enforce a specific buff window you want to line up your burst with, but you can also make maintaining this buff a core part of the job itself. But it doesn't have to stop there. Things like Monk's Brotherhood and Reaper's Arcane Circle can also be used as another type of raid buff. Just remove the damage portion and this still allows other members of your party to contribute to your own damage. This could even be extended to a debuff to increase damage.
Going onto synergies, if we are looking at things like the damage resists (slashing/piercing etc.), You can either design a job assuming it is always going to have it, which leaves it underpowered when that debuff isn't there, or, you tune it when the buff isn't there, which makes it overpowered when the buff IS there. You could, in theory, stick it somewhere in the middle, there has to bee a sweet spot after all, but you also have to remember you are potentially fighting for that spot from another DPS, who might not have that issue. Then there is slashing specifically, Paladin and Dark Knight have magic attacks, which do not benefit from the slashing debuff, so how do you balance those with Warrior and Gunbreaker where everything would benefit from slashing. It is more variables that really don't contribute to a jobs enjoyment.
With the removal of the 2 minute meta, every job should undergo changes, whether they are small changes or more drastic, it doesn't matter. For example, re-imagining Monk as a more sustained DPS that has short burst phases based on Perfect Balance and not Riddle of Fire or Brotherhood's damage increases, the idea here being, with short, more flexible burst windows, you have more choice as to when you burst and so you are adapting to each fight, much like a martial artist would adapt every time they fight. I have actually been playing around with this concept in my head for a while and have done a rough google doc outlining my idea. It is mainly just to get it out of my head so I don't know if I would even release it, but it can be a fun thought experiment to re-design a job based on some sot of environment you have crafted.
For GCD speed, it is worth noting that currently, the quickest any GCD that has a variable GCD timer can go is 1.5 seconds. Some have an issue with 1.5 seconds just to single weave, which is mainly highlighted by Machinist, but it could also pose a problem for Reapers, maybe even Viper if you stack enough Skill Speed. That said, there are quite a few oGCDs that jobs have to juggle to fill in the time between the GCD. so you don't really feel it. Could more jobs across the roles be quicker, for example, a faster GCD tank? Sure, but a universal shortening of the GCD just wouldn't go down well I don't think.