The most common mod for almost any headphone is pad swapping. Some headphones, like the Hifiman HE6se V2 have such awful stock earpads that almost any other pad makes them sound better and more comfortable. Others, like most Sennheiser headphones, will change the sound for the worse with aftermarket earpads.

EDIT: Also EQ. Some people prefer to EQ to find their preferred sound. I personally dislike heavy EQing. Always makes my headphones sound boring and similar. And I always have to change something when I change something else, similar to physical mods but not as permanent.

But I do like to apply a small +3db bass shelf whenever I’m yearning for that nasty bass. Not always though lol