Generally speaking, I would still keep the SOUL of the "Classes" alive, but mix them up a little bit. When you start the game, instead oif the Merchant-brother you interact with (Brendt, Brennan, or Bremont) handing you a Ring, they instead have you your first Job Crystal -- except it appears as just an ordinary piece of stone.
Just add a single line of text (because none of them actually speak), where he says something like "Speaking of <city>, I found this stone outside the <starting job>'s guild on my last visit. I thought it might bring me luck, but I've a feeling you should have it."
In addition, when picking up any of the "Classes", you're given a similar stone, with similar text like "The guildmaster would also instruct you keep this stone close to your heart."
So instead of being GIVEN a Job Crystal, you instead AWAKEN the one you already have! So mechanically, it's the same except you're "your job" the whole time, completing the job quest just gives you the proper title for said job.
Second, I would add "Job Crystals" for your crafters, as well. Really, I'm just not convinced there's a compelling reason NOT to, considering Shadowbringers already added Crystals for them. If anything, they deserve nicer-looking icons, imo.
NOW...
Some people feel like "Job Crystals" should be removed entirely. I'm honestly okay with that, in terms of items that take up inventory space. I still like the concept of them, but that needn't require an armoury chest tab.
HOWEVER...
If you keep Job Crystals, I would personally make one seemingly-minor change. When not wearing ANY Job Crystal, you default to a simple "Adventurer". As this "job", you only have access to general abilities, but can equip any item in which your "level" is high enough. The purpose essentially being, for taking screenshots of otherwise impossible gear combinations.
Your Adventurer "level" would instead being an average of all other jobs, rounded down (jobs not acquired would count as "lvl 0"). Not much point, right?
But borrowing inspiration from the "Onion Knight", maybe once you reach max-level with ALL jobs, maybe it becomes a sort of "Limited Job", skin to the Blue Mage, useful purely for solo'ing old content? Or heck, maybe it's treated as even a sort of "joke class", where it maintains zero abilities, but its basic auto-attack becomes incredibly powerful, again just for non-relevant content?
Is there a lot to gain from this idea? No, not really. But I dunno; something about the idea of this sort of "secret job" that everyone has access to, just sounds kind of fun. Heck, maybe it could even play some kind of role in unlocking some special glamour(s), just having you revisit a bunch of folks you've helped along the way, and maybe some kind-of-dumb "challenges" like, "a special Alliance Raid with only Adventurers, basically auto-attacking and maybe a jump-puzzle DURING a boss fight"?
I dunno. Just the kind of wild ideas that run rampant in my brain!