Okay, want to revamp the first suggestion slightly
Caster Bonus
Caster Bonus is a measure of a PC's supernatural power and control. Recorded on the party card, it is based on the attribute bonus for the PC's casting attribute. This INT for Wizards, WIS for Cleric, Druid, and Ranger, and CHA for Bard, and Paladin. If the player increases the appropriate attribute, the Caster Bonus increases as well. For example if an Elf Wizard equips a Token that gives her a +4 bonus to her INT, her Caster Bonus increases from +3 to +5. There may be other ways to permanently or temporarily increase your Caster Bonus, besides just increasing your Attributes.
When a Cleric, Druid, or Wizard casts a spell that allows a Skill Check, if the Skill Check succeeds, the damage or healing of that spell is increase by the PC's Caster Bonus. Caster Bonus does not have an inherent effect from Bard, Paladin, and Ranger, but it may come into play in other ways.
Then to add a bit to the above:
Amulet of Inclination
+1 to Caster Bonus
Usable by: Bard, Cleric, Druid, Ranger, Paladin, and Wizard
Bracers of Intent
Damage bonus to Favored Enemy becomes your Caster Bonus
Usable by: Ranger
Tome of Vibrato
While singing, if at least one melee attack hits a monster, it takes Sonic damage equal to your Caster Bonus
Usable by: Bard
Xavon's Mythril Trumpet
Party damage bonus from Bardsong becomes your Caster Bonus, and Bardsong applies to spells
Usable by: Bard
Librum of Divine Barricade
Prevent damage to one other player equal to your Caster Bonus (1/room)
Usable by: Cleric, Paladin
Ring of the Cabal
Increase your Caster Bonus based on the number of Rings of the Cabal in the party. 1: +1, 2-3: +2: 4-5: +4, 6+: +4
Usable by: Bard, Cleric, Druid, Paladin, Ranger, Wizard.
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