now I tend to very few changes to any game system I play. Change settings more than systems.
1st Ed AD&D we added in that Rangers get, same as Rogues, Move Silent Hide in Shadows only in the woods, etc.
Added in that you could take Ranger view Fighter on the way to Bard. Those were it I think.
None for 2nd Edition other than trying to smooth out Secondary Occupation a little.
3.0/3.5/Modern I just blend
Hero System is stock as is. Only limits are soft such as starting out no more than about 20 total Defenses, keep it appropriate etc. More a touch of the playing group dynamics than anything else.