Well they can do Geomancer with Conj, and the reason for that is Stone and Aero, both of those are considered a norm part of a geomancer. Cleric stance will allow them to concentrate on dps spells, and they can get other skills and abilites like Flood and Tornado and Quake from their soul crystal, also they can make it so the Geomancer, like Summoner, could help healing but won't be effective due to stat build and traits given to them.

As for Gladiator, they can become Dark knight, they can get a trait to remove enmity off their attacks, and it can work with some life draining attacks, and poisones and things like that attached to their soul crystal which will change certain combos to do something else.

Thaum, why not make them a healer with their secondary job? And how do you do that, a Sage. Their crystal gives them access to cure spells, but to make them a different kind of healer.. make them a dps healer.. meaning they get their heals and mana according to astral fire and umbral ice, so they will still need to cast fire and ice spells. Ice will increase their mana restoration, while decreasing the potency of their heal spells, fire will reduce mana restoration while increasing the potency of their heal spells, so they will become a class that needs to know balance.

While Archer, yes it would stay as a range dps, but you can make them maybe become Ranger, a pet class archer? or maybe give them elemental arrow shots and in essence a caster? There are plenty of different things that can be done to change up how the archer works.

Puglist, well Dancer does work due to the different forms with their attacks, but a tank dancer? How about instead a melee support class. Basically Bard is range support with songs that help increase party performance, maybe dancer can while in different forms for attacks cause certain debuffs to the mobs or even a form for tp restoration?

Marauder, well Besereker kinda works with them. Give them an ability to remove enemity, and attacks and stuff to increase their damage output, but while damage output gets increased, they lose defense or certain abilites cause enrage in which they lose the ability to move... things like that.

Lancer, in my opinion is the hardest, and that is because there is no other job in final fantasy lore that uses a lance/spear. I've heard people keep saying dark knight and sam.. but the limitation is the weapon. to use a different weapon other than the lancer/spear, it will need to change the animation of attacks, and that is something I don't see happening.. so I'd be interested in seeing a 2nd lancer job.. but if I had to create one.. Pikemen.. basically a tank with a spear and shield.