Hidden Secret Ending:
After defeating the final boss of the main story you can unlock a hidden secret ending by collectng ALL the secret reports in the game, this also inlcudes the one in 'Another Day'.
Unlock Chapter Selection:
Beat the game once to unlock 'Chapter Selection' on your 'Phone' menu. 'Chapter Selection' mode allows you to return to any part of the game and replay it with bonus objectives.
Unlock Ultimate Difficulty:
You will be able to purchase this feature as a quest item when you go to the Shiba-Q Headquarters on the 3rd floor of 'A New Day Chapter'.