Here is a pretty simple idea that I think will be a home run if everyone takes a moment to read what I have to say.
My Proposal: Instead of creating support jobs based off of the LV. 99 version of a job, I believe that there should be completely new Jobs that function only and exclusively as support jobs
Support jobs have always been based off of the halved version of a real job
ex.) PLD 99 --> PLD 49
PUP 99 --> PUP 49
SMN 99 --> SMN 49
But, in a lot of cases, just because a job can be used as a support job doesn't mean it necessarily is useful as a support job. The examples shown above are jobs that are capable of being used as support jobs, but are not by any means optimal in comparison to other support jobs. They still can be useful in certain situations, however.
The point is Square Enix, I don't believe, creates all jobs equally with the intention of them both being useful as a Lv. 99 job and a support job. /WAR /RDM /NIN are all great examples of useful support jobs, but /BST /GEO /PUP can all be very questionable support jobs if you're trying to be useful in end game content.
If we implement job classes that are specifically meant to be support jobs, then we can cater to different jobs classes that don't have support jobs that necessarily benefit their job.
Here are some examples
Support Job: Medic - This support job can completely unique job abilities and spells from all other healing classes that help specifically with healing and conserving mp.
Support Job: Viking - This support job can focus on defensive job abilities, job traits, and spells that benefit close line jobs and tanking jobs like Paladin and Rune Fencer.
Support Job: Herder - This support job can give job abilities, job traits, and even spells that are especially beneficial for all pet jobs including Dragoon, Beastmaster, Puppetmaster, and Summoner.
Support Job: Berserker - This class can focus on job abilities and job traits that enhance front lines damage dealing jobs including Monk, Samurai, Dark Knight, and other classes. Job traits could be things like dual wield, enhances acc and attack
These types of support jobs can be "General" type support jobs that benefit a variety of job classes while also being a little bit more focused on certain types of fighting whether that is magic, meleeing, or tanking.
The idea behind these support jobs are that when they're unlocked, the support jobs do not need to be leveled, they can be immediately used and are always half of your main classes level. This means that when these new support jobs are implemented, no man time is necessary to create job specific gear or quest lines that go with these classes. They could be unlocked by purchasing the class from a NPC or doing a simple mini quest which would allow Square Enix to implement a lot of new support jobs that benefit specific jobs and class categories. The support job classes are meant to only reach level 49 (or whatever half the level cap is at the time.)
A next class of jobs exclusive to support jobs could be some of the more popular selections that the player base ask to have to have implemented as real jobs.
Support Job: Time Mage - Grants access to spells such as haste II and or hasteja, Slow II and or Slowja, Reflect, Gravity/Graviga, Stop/Break and could carry innate job traits such as Haste 5~15%, movement speed +18%. All sorts of fun stuff.
Support Job: Necromancer
This support job can give access to spells like death, high level dark magic, and other new black magic exclusive to its job.
Support Job: Onion Knight - Grants access to a large variety of job traits and job abilities such as provoke, sentinel, waltzes, meditate, steal, and even access to more general spells like cures, enspells, protect, shell, sleep (of course this could be powerful as a support job, but just an example of how the job class works). The job abilities and spells accessible at a given time could be similar to how blue mage sets their magic spells, so that you can't have every job ability from every class on at the same time. And the job abilities accessible can be more general so that other support jobs are obviously still useful.
Anyway, I think by implementing exclusive support jobs, players can diversify their support jobs depending on the situation and not necessarily be stuck with support jobs like RDM, WHM, WAR, NIN, DNC. Jobs like Puppetmaster, Beastmaster, Geomancer, Scholar, Bard that don't really have support jobs that really fit their job can have more options. And even jobs like Paladin who typically support /RDM, /War, or /NIN can use new support classes like /berserker to help them damage deal better in cases where they are soloing.
Square Enix can also implement new and useful support job classes without really worrying about putting too much man power into it. All they would have to do is create a few new job abilities and spells along with possibly some mini quest.