I've been thinking a litle about the 2-classes-each thing too, purely as an "I'm bored, this would be nice" exercise.
We could do with a few more Ranged damage dealers, another Healer, and a Tank that can mitigate Magic damage reasonably well.
Plus a lot of people have been crying for DRK, THF and RDM to be added to the game... THF in particular could be very useful for helping with Hate management (like FFXI's Sneak-Attack/Trick-Attack setup!)
I paired Berserker in the same place as you, but I'm not sure that DRK (primarilly a 2-handed squishy DD role) could replace GLD (as primarilly a 1-handed tank role). I think it could work out quite well under LNC though.
*** Disciples of War ***
GLD --> Mystic Knight
Requires Thaumaturge (Elemental Damager Dealer) to give a Tank class which can enchant their blade and armor with elemental magic.
[This would grant appropriate bonuses to attack and defense. Think "Enfire"/"BarFire". More focused on countering Magic Damage than PLD]
MRD --> Berserker
Requires Pugilist (Unarmed Stance-using Attacker) to give enhanced Damage output (via some form of enraged or "Hulk SMASH!" stance), at the expense of HP/mitigation.
[The 'Damage Dealer' version of Warrior. Able to off-tank weaker foes in a pinch, but mainly focused on sustained DPS for chained-encounters or long boss fights]
PGL --> Thief
Requires Archer (Ranged Damage Dealer with increased runspeed) to give a class that can use both melee and a little bit of ranged damage (daggers?)
[Evasion-focused DD. Would have positional attack buffs (remember 'Sneak Attack'/'Trick Attack' from FFXI? That sort of idea!) and Steal/Flee]
LNC --> Dark Knight
Requires Thamaturge (Magic Elemental Damage) to give a Two-handed Melee Attacking Magic user.
[A few Darkness-themed nukes plus Drains, Stuns, Sleep, Etc for utility. High Melee Spike Damage (would tie in with Lancer Limit Break) whilst wearing quite heavy armor]
ARC --> Tamer
Requires Arcanist (Pets and Support) to give a Damage Dealer which can capture certain Monsters. Weaker monsters (think FFXI's "Jug pets") can be summoned in instanced missions.
[To keep this viable, Tamers cannot equip the same bows at endgame as Rangers and Bards, instead their weapons will grant additional Pet-Related Buffs rather than extreme amounts of Additional Damage.]
*** Disciples of Magic ***
CNJ --> Red Mage
Requires Gladiator (Sword + Shield Melee Attacker) to give the typical "Jack of all Trades" character we all know and love*.
[*Realistically in order to make this viable, a Red mage should be able to function as a Ranged Healer, Ranged Nuker or Meleeist. But not all at the same time. The Int|Mnd swapping stance should be kept, and a Mnd|Str stance added. The user could therefore choose whether to Melee (-->Str) Nuke (-->Int) or Heal (-->Mnd). As a fighting-focussed mage class, it'd make sense for them to have Party-AoE Phalanx and Barspells - this would keep them more competitive at endgame due to unique buffs. They should have less raw healing potential, but be more hardy than a WHM.]
THM --> Blue Mage (Or maybe Geomancer, I like the idea of Elements/Weather manipulation!)
Requires Arcanist (Support and "Arcane Magic") in order to give a character that can learn certain (Arcane?) Enemy Abilities.
[Unlike FFXI this class would NOT be able to offtank or melee competitively. It's mainly a caster DPS role with some added extra utility (Buffs/Heals/etc)]