Skill Specialty Dice

Rather than (or in addition to) adding a flat number to your rolls for proficiency you add a die, which increases in size per your expertise: d4, d6, d8, d10, or d12.

This allows a little more flexibility than just double proficiency or similar, while boosting the average die result by 2 to 6-ish, ascending.

Because the average added is so variable, it may be best to make the applications broad. For a feat it ahould grant expertise on one skill, for a talent, two.

The bonus is training, just like the flat bonus to the skill. Where your training would not apply, neither does your expertise.

These could be narrowed into specializations for each skill, such as lock-picking, pick-pocketing, or traps for Thievery; or beasts, plants, spirits, and geography for Nature; or etc.

This narrowing of scope both allows for a little more PC flavor (one guy's really good at deciphering writing, but absolute pants at the rest of Arcana, e.g.) and for controlling power level (because adding a d4 is almost always preferable to just a +2; there's only a 25% of it being worse, and a 50% of it being better).

Could use this same system for saves (vs poisons, charms, fire, whatever) or even attacks (individual weapons w/in a group: rapier, or guisarme) but those lend themselves more to power-gaming in combat. Power-gaming in non-combat can be problematic, but typically far less so.