THM could have turned in to warlock(WLK?) and BLM.
ACN should have gone SCH and some other dps based on sacred geometry.
SMN should have been it's own class, not this odd version of spiritmaster we got.
ROG could have easily gone NIN and THF, but we got a weird excuse in the form of "thieves are bad so no one would want them in their cities, etc. etc." People say "dress your nin up like a thf!". Thieves do not use ninjutsu and do shoryukens.
MRD could have an option to set down their axes and pick up a pistol to become Musketeers at level 30.
ARC could have easily gone BRD or RNG at 30.
GLD could have gone PLD and Russel Crowe (jk) Dual Wielding berserker of some sort.
PUG could have an option to pick up a staff at 30 and choose to channel their Ki/Chi to heal people as opposed to punching people. Or even had a tanking "spec" instead of DPS.