Quartz
Hero
Would you be okay with class abilities that explicitly addressed multi-classing? Something along the lines of
Cinematic Defence: a single-classed Fighter may use her Proficiency Bonus instead of her Dex bonus when calculating AC; a multi-class Fighter may only use a +2 bonus instead of her Dex bonus.
Divine Agility: a single-classed Paladin may use her Charisma bonus instead of her Dex bonus when calculating AC; a multi-classed Paladin may not.
Cinematic Defence: a single-classed Fighter may use her Proficiency Bonus instead of her Dex bonus when calculating AC; a multi-class Fighter may only use a +2 bonus instead of her Dex bonus.
Divine Agility: a single-classed Paladin may use her Charisma bonus instead of her Dex bonus when calculating AC; a multi-classed Paladin may not.