Halo 4 has, for the most part, been devoid of challenge or little hidden secrets (Unless you are still doing the 3 Mystery Easter Egg Hunt thingy, or Section 3).
I, for one, would love to see something like the Vidmaster Achievements come back. Something that you have to team up with friends or community members for in order to unlock something interesting (Halo 3’s was Recon, Halo 4 could have something else).
It’s not something big, but it’s just something extra that is fun to do, along with possibly having people go back to Campaign missions.