Halo 4 rejected map pitches
From Halopedia, the Halo wiki
During the development of Halo 4, company Digital Extremes was contracted by 343 Industries to help with the production of the game's DLC map packs. A number of map ideas and concepts were pitched to 343 but went unused, and have since been released online.
Known map concepts
Covenant underwater technology
Identified in a series of concept art uploads all titled
Last Resort remake
One map concept shown involves a paintover of the sniper tower area of the Halo 3 map Last Resort. Its filename,
Other DLC map concepts
Some callout sheets for individual map props have been posted online, of which tie to no particular known map.