One of the things I loved most about old Rareware games was all of the hidden, unused content either found in the cartridge's coding or briefly mentioned in pre-release material (I still have a video of Beta-Banjo stuff up on Youtube somewhere). I'm talking about things like levels like Fungus Forest, Mt. Fire Eyes and the Giant's Lair, or unused music like the infamous Advent or the beta themes for Click Clock Wood and Treasure Trove Cove/"Hammerhead Beach", or even unused game modes like Bottle's Revenge, least we forget Stop N' Swop.
True, a lot of these things may end up getting cut in favor of touching upon them in a sequel, but some content gets cut for good and never mentioned again. Now, Insomniac Games had secret areas in some of their Ratchet & Clank games that served as museums for scrapped models, items and concept art for their games, so I think it would be amazing if Playtonic did something similar. It would be fun to take a break from the new adventure and search for a hidden place like this in order to get a grasp of how the game proceeded through development as well as take a gander at some glorious concept art.