Hmm, does the multi-class have to synergize in power?
Marauder with Sneak-attack Carnage etc?
Two classes strengths can also complement each other. Give options/utility.
Or like a Holy Slayer, Rogue-side gains survivability, Paladin-side gains damage dealing potential.
Or Geomancer, Ranger is great for Single target sniping, but Wizard is better for AoE and debuffing in general. But no real ranged damage booster.
For instance playing a pure priest has a lot of downtime with few casts and few decent spells to cast.
My merc, Mystic of Wael filled that downtime generating focus with a Warbow, which provided some nice CC that the Priest on its own could not provide.
Cleric of Wael felt a lot weaker even if a bit more sturdy.
But a Shifter Oracle get a lot of Penetration + Base Dmg from its forms, which should generate tons of Focus to spend, perhaps on that Soul Blade thing.
A Hierophant could play like a Battlemage, Mirror Image + Chill Fog would allow the Cipher to safely walk the front line with 2 sabers in hand, which would most likely generate more focus than a pure Cipher would dare to do without starting at range.
You also have a limited number of slots in your party. 5 slots with 11 classes.
Even if we use multi-class mercs to cover as many classes as possible, we would still need to leave out one class.