FrogReaver
The most respectful and polite poster ever
I dunno.
Fighter Level X (whatever level you want to set it at) - When determining the DC of any Skill Athletics or Acrobatics check, subtract 5 (then 10 later on) from the DC for this level fighter. Any Athletics or Acrobatics skill check which achieves higher than 30 result allows the fighter to perform a legendary action as determined by the DM. Possible examples might be racing up the side of a mounting, running across a river, lifting an entire sailing ship out of the water.
Seems fairly simple to add something like that into the game.
Maybe - but shouldn't it also apply to at least the barbarian? What about the monk? The ranger or paladin? Why shouldn't it apply to a rogue? What about an extremely strong wizard?
That is - if it's really just an ability that is mundane - then the option for something like that should be available to quite a few classes.