I have recently installed ExtraHardMode. It would be nice to have a multiplier to reflect the increased difficulty. For example:

 # WorldDifficulty
  world-difficulty-multiplier:
    difficulty:
      multiplier:
        peaceful: '0.5'
        easy: '0.75'
        normal: '1.0'
        hard: '1.0'
        extrahard: '1.5'

I suppose that would require integration, or at least recognition that EHM was configured in a particular world. An example of the relevant configuration node is

ExtraHardMode:
  Enabled Worlds:
  - world_yellowdog

Released v4.8.6