On the subject of Dark Knight:

A common trend in most RPG lore, as far back as AD&D at least, and included in Final Fantasy lore as well, is the duality of Paladins and Dark Knights. In Dungeons and Dragons, Blackguards (evil Paladins) are either seen as an advanced class for Paladins or as a special kit for Paladins who wish to be evil to take on character creation.

In Final Fantasy, the most famous Paladin in the entire franchise, Cecil Harvey, started out as a Dark Knight and became a Paladin. I simply cannot see Dark Knight coming from anything else than the class that spawns the Paladin simply for that kind of Duality.

------

On the subject of other Second Jobs: Marauder

Marauder's second job will obviously be the DPS-heavy Berserker. I simply cannot see it being anything else anytime soon.

------

On the subject of other Second Jobs: Lancer

There are a few other jobs who have been able to wield Lances and Spears. While it is a stretch, Edgar Figaro from FFVI was a combined Lancer and Engineer. Therefore, I think it would be reasonable to let Lancers become Engineers this way.

Also, in FFXI, the Samurai Job can also equip spears, and in FFTA, the Templar Job can also equip spears. So there's a couple thoughts.

------

On the subject of other Second Jobs: Pugilist

The most likely one I can think of would be Black Belt, which would be much more like a traditional Final Fantasy monk in that it would have much higher HP.

------

On the subject of other Second Jobs: Archer

The most obvious choice would be Ranger, but other options could include FFTA's Sniper.

------

On the subject of other Second Jobs: Conjurer

There are many different mage Jobs that utilize White Magic or something similar. However, considering the Conjurer's direct ties to nature, I am willing to bet that Geomancer will be the most likely Second Job for the Conjurer.

------

On the subject of other Second Jobs: Thaumaturge

There are many different mage Jobs that utilize Black Magic or something similar. However, with how Astral Fire and Umbral Ice work, I wonder if a twist could be put on those abilities and work well as a Time Mage, changing Astral Fire to Astral Haste to amplify spell speed (rather than increase damage) at the cost of increased MP, and Umbral Ice to Umbral Slow to regenerate MP quickly at the cost of reduced spell speed?