Well back in early 2.0 my thoughts at the time were that each of the base jobs should have had 2 soulstone paths at 30 that followed different job roles like they did with Arcanist going into SCH and SMN but with HW they moved away with that on the new jobs. Granted those jobs weren't available to players starting a new character at lv 1 and could have been called "advanced" jobs due to it. However I think it could have atleast at that time helped with the dps queue times issue back then to a degree by essentially giving all DPS jobs a path or option to play and fill another role. It could have created issues with poorly equipped healer/tank though or having a healer/tank that isn't very good at playing the role so it wouldn't have been without its own drawbacks. It would have atleast presented players with an option though.
Heavesward job role distribution over playerbase didn't change too much that I recall because they released a new job for each role. However when Stormblood launched we only received 2 new DPS jobs. Due to this initially the ration shifted considerably to essentially an overpopulation of DPS players as people were trying out the new jobs which resulted in all the issues of DPS players complaining about 30+ minute queues for almost everything in duty finder until the newness of the new jobs wore off and the role distribution returned to what it was before SB.
Looking at the jobs we currently have there is a few things I think we could use to flesh out he job selection in general
Another tank that actually uses a shield. Currently at end game only PLD even makes use of that gear slot.
Tanks that deal either piercing, blunt, or magic damage primarily. Currently all 3 deal only slashing damage.
Another heavy melee DPS, Currently only DRG occupies this role and as such gets an armor class all to itself
Another dex based melee DPS, NIN currently is in the same situation as DRG in terms of armor.
Another DPS that deals blunt damage
A ranged physical DPS that doesn't deal piercing damage
A true magic damage melee DPS
A healer intended to be up front an in the mix rather than one that sits back and casts from the back row.