Like what has been announced for GH5 [1].

Challenges that are song-specific could bring a whole new dimension. Unlocking items does not really hold for FoFiX tho.

See the reference for more details.

[1] http://www.destructoid.com/exclusive-guitar-hero-5-challenges-revealed-140210.phtml

We could have "trophies" or "medals" awarded for each successful completion of a challenge. That way it wouldn't effect scores in any way.