They can keep the same difficulty but make a 8 men version of it. Since small guild cannot get 16 all the time.
What does it have to do with difficulty?
Then you will have people wanting a 2 man version of HM TFB because their guild can't get 8 people online.
This encounter is designed for 16 people, you can always pug the ones you can't fill with guildies, but asking for an easier version so 8 people can do it is not the answer.