I just watched the new Video from a D&D content guy I like Called The Dungeon Coach, He is big on home brew and customizing D&D. Check him out if you like D&D as well.
His video was a chart he made to quickly add abilities to your bad guys or Monsters to help make them more unique, or to adjust the fight on the fly if it is going too quickly. This made me think what are some good abilities to give NPCs to help make them more of a challenge and to change up the fight.
Let me know if there are some you have used often or some that were only for the BBG in your fights.
Here is a link to his video to see what I was talking about:How to Homebrew Monsters in Seconds | Speed Up D&D Monster Game Prep - YouTube