Why do you think you have to change the rules? I think you need to change nothing.
I dunno:
*Maybe I want a world with only ritual magic or a world where clerics don't exist but wizards have cleric spells or a world where cantrips don't exist or where magic exists but is incredibly dangerous to use.
*Maybe I want a world with herbal healing or where every housewife has her own potion of healing recipe or where hedge magic is common but archwizards are impossible to find.
*Maybe I want a world of knightly orders in plate going about questing and such and would like to mechanically differentiate them to hit home on their ideological and thematic differences.
*Maybe I want to give fighters options other than hitting something with their sword every attack. Maybe I'd like to see high strikes, lunges or hamstrings given some mechanical weight for a dash of tactical complexity.
*Maybe I have a player that really wants to play a half-giant luchadore and suplex some ogres.
*Maybe I have a player that wants to play a viable high-INT, high-WIS, high-CHA fighter.
*Maybe I have a player that wants to play the farmboy on a grand adventure trope.
*Maybe I have a player that wants to play a fighter, but would like to trade out action surge for more consist round-to-round damage.
*Maybe I want to add depth and complexity to the choice of arms and armor--maybe adding strength requirements or different advantages of an axe vs. a sword, for example.
*Maybe I find the guidelines of acrobatic maneuvers, or tracking or swimming or getting through high tea without making a faux pas a bit lacking.