Add specializations to all the jobs. My ideas for the casters are as follows:
SMN - One "standard" spec that plays more or less the exact same way it currently does. One "big pet" spec that replaces the egis with more demis, extends the duration of the demis, and adds additional buttons to Bahamut and Phoenix. One "small pet" spec that replaces the demis with more egis.
BLM - "Fire" spec that plays the way BLM currently does. "Ice" spec that deals less overall damage than Fire, but extends the Astra/Umbral timer and doesn't have MP to worry about (this is the "beginner" spec). "Lightning" spec that's proc-based like ARR BLM and full of instant casts (this is the "pretending to be a phys ranged" spec).
RDM - "Balance" spec that plays the way RDM currently does, but Verraise has a lengthy (multiple minutes long) cooldown. "White" spec that lowers overall damage while buffing Vercure, removing Verraise's cooldown, and adding additional defensive utility. "Black" spec that increases overall damage while disabling Vercure and Verraise and other defensive utilities.
All the healers get a "DPS" spec (Geomancer for WHM, Chemist for SCH, something else for AST and SGE) with a complex rotation, no oGCD heals, and passives that cause GCD heals to generate the same resources for the big burst buttons that the DPS spells do. Basically, they build resource whether you're DPSing or healing, with the caveat that your oGCD heals are disabled entirely so you HAVE to use GCD heals in order to do your job. They all additionally get dedicated "pure healer" and "shield healer" specs to add more party comp options; the "DPS" specs offer primarily pure healing for WHM and AST, and shield healing for SCH and SGE, same as present.
All the tanks get "self heal", "mitigation", and "DPS" flavor specs, where the self-healer specs have strong self-healing (basically current tanks), mitigation specs have no self-healing but strong cooldowns, and DPS specs have neither of those things and no invuln either, but deal a bit more damage than usual and have a more complex rotation. The intent here is for self-heal specs to be used for dungeons and soloing stuff, mit specs to be used for progression raiding, and DPS specs to be used for offtanking and trivial content. As long as SE tunes bosses right (meaning, attempting 4th-Floor Savage with 2 DPS tanks should result in both of them getting one-shot by the first buster without heavy support from the rest of the party, regardless of their gear situation), it'll work just fine.
Phys range should all get one "standard" spec (basically the way the class plays now), one "selfish" spec (less team buffs, more damage; MCH in particular gets cast times on all its buttons in exchange for its DPS going through the roof), and one "unique" spec based on the class. BRD gets a DoT-based spec, DNC gets a movement-utility spec (basically, lots of dashes + positionals that debuff the target), and MCH gets a permanent pet spec.
No idea what to do about melee DPS. I don't play them much so I have no ideas here.
Basically, if you like your current job, you can keep your current job, and the other specs are supposed to add variety to some of the DPS, fix long-standing issues with others (looking at you healers, resmage, and the perpetually-undertuned MCH), and generally freshen the game up a bit. The DPS specs for tanks and healers in particular have very intentional weaknesses that are supposed to keep them from being 100% meta 100% of the time; if a dungeon party forms with a DPS-spec tank, a DPS-spec healer, and two selfish-specced DPS, the intended outcome is that they either get stuck single-pulling, or someone has to respec, because wall-to-wall pulls in that situation should consistently result in a dead tank from lack of sustain. Alternatively, if the tank goes self-heal spec and one of the DPS is a White-spec RDM keeping Verprotect (just an example) and other mits up on the tank, the healer and other DPS should be perfectly fine doing selfish DPS things for that run. And of course, if all four players are using the "standard" spec for their jobs, they'll naturally wind up with a balanced party and be able to play the game the way they already do today.
(And preferably, players would be able to respec even in instances, as long as they're out of combat.)