I do see the thematic appeal of GLD -> PLD or DRK, but the weapon is the sticking point for me. DRK needs a f**k-off big gsword or scythe. Plus MRD main with THM "sub" screams DRK as well.
I think, with what we've seen, the class needs 2 distinct aspects, that each job specialises in. Of course (like MRD+THM above), sometimes it is just the combination of classes that creates the job. Some other potential ideas for 2nd jobs, with relevant level 15 "sub" listed:
ARC - nailed it
MSK - same. MSK needs gun + minor buffs. COR (ARC sub) builds buffs, GNR (LNC or MRD sub) builds damage
GLD -> Fencer (ASN sub perhaps - see below) (emphasis on DD aspects of role)
MRD - see above
PUG -> MNK or DNC (ASN sub) (evasion/elemental aspects of role perhaps)
LNC - tough one. SAM can certainly use polearms historically, but a) it is DRG signature weapon, and b) GKatana. Argument won :)
CNJ - not bad
THM -> Illusionist (ACN sub). AoE damage, plus (pending SCH skill set) battlefield/elemental control
ACN - given.
I also think (but we do have 9 classes, so 1 more is odd) that THF & NIN can easily branch off from a common class. Dual-wield, high evasion, some basic ninjitsu-type "magic" for escapes etc. THF (LNC sub) gets dmg boost + positional attacks, TH etc. NIN (PUG sub) gets elemental/debuff ninjitsu + shadows (possibly. They can't break the game like XI, given it isn't a subjob). Is Assassin too much for a class, as a name?