If jobs are for party play I'd say the quest to unlock would be anywhere from lvl 20 (still during exp multiplier) to level 35. Since jobs arent supposed to be solely for raid material but also for grouping/exping (especially with the new exp reworks otw). I'd expect them to come at a level so the player can experience their use otw to 50.

While I'm sad it is only one job per class atm, I look forward to their goals in the future. Classes are being reworked in 1.19 and 1.20 some more before jobs are released in 1.21.
my guesses:

lnc@30 = dragon knight (we may not be using dragoon since ishgard has them and they use any weapon and seek to eliminate dragons, although this is a diff FF and can use a new iteration)

con= whm? would be my guess, just sucks tho cause in storyline con deals with the elements

thm =blm? again these two are having their skills reclassed

gla = pld (confirmed, though I wish it was shield so they could have included another job on gla)

mrd = ?

arc = brd? it was mentioned that brd is confirmed so idk what class it would go on (many mmo use ranged for buffer) dun dun dun...

Its the future I'm most interested in. Multiple jobs, cross class jobs, etc (I still wish you could equip jobs on more than one class, obviously nothing too silly, my personal wish is GKT with drg play)