I think a great addition to Halo 4 would be a modified invasion gametype.
It would be like objective based gametypes specific for certain maps. For example, A team of Spartans would be in a UNSC Frigate, flying above a Forunner/ Precursor Ship, then they would drop down in drop pods, retrieve enemy intel or something similar such as planting a bomb, then evacuating the ship by pelican and returning to the firgate.
There are many possibilities on different objectives and different maps.
I think this idea, if tuned right, added to and balanced correctly, could be a great addition to Halo 4.