Strange talent

[Evoker]

Prerequisite: Must be able to cast spells

Benefit:

When you choose this feat, gain a +1 to any mental ability. Whenever you cast a spell, your spellcasting ability is treated as being the ability you improved with this feat.

Example:

Thordon, the gnome cleric, chooses to improve his Intelligence. Now, when he casts a spell, he uses his Int modifier whenever a spell refers to his Spellcasting Ability. He later multiclasses into sorcerer. He still uses his Intelligence for his spellcasting ability whenever casting sorcerer spells.

(I built this to replicate the classes in 3.5 that were "divine caster using CHA" "Arcane caster using WIS" etc.)

[Fire Tarrasque]

That is a powerful ability, but probably not totally overpowered.
You did good.