Arcanist can split into Summoner and Scholar, both wielding books for their main hand. This idea builds on that for the other 14 jobs.

Marauder splits into Warrior (tank) and Viking (dps). Viking wields two axes and instead of defiance and deliverance, it gains a Carnage gauge that increases attack speed and be spent on abilities. The Viking would dual world axes.

Gladiator splits into Paladin (tank) and Cleric (healer). The cleric wields a mace and uses a combination of instant cast heals and proc heals, similar to excog. Melee dps between heals and perhaps a beneficial group aura.

Dark Knight (tank) would have a split job into Reaper (dps) and wield a scythe that has a darkness pet to fight alongside him. The scythe would have a longer melee range, giving it more mobility.

More ideas in following posts