Quartz
Hero
I'm thinking along the lines of the Battlemaster Fighter or Monk here. Call it the Disciplined Warrior. At some point, perhaps instead of getting Indomitable, the Fighter gets the ability to substitute their Proficiency Bonus for any ability modifier 1 or 2 per Short Rest, at will and for one roll only or to the end of the round (note, not the end of the Fighter's turn), whichever comes first. So the PB +4 Int 10 Fighter could apply that +4 to their Int save if they were attacked by an Intellect Devourer. It would not just apply to saves but anything. E.g. a low-Cha PC could apply it to a social roll. Or even a low-Dex fighter having to pick up a bow. To prevent multiclassing cheese you'd have to limit the bonus to the lower of the PB or levels in the fighter class.
Or perhaps this might be better as a Feat?
Or perhaps this might be better as a Feat?