Urban Legend of Zelda: When the game was just coming out, the lack of a Legendary ending and apparent confirmation from Bungie in a commentary video that there IS some sort of bonus ending lead to massive amounts of speculation on how it may be unlocked.
A whole mess of rumors surround the Data Pads littered throughout the levels, as well as hidden switches off the normal paths.
And technically playing on Legendary does change one thing: Keyes refers to Halsey by her first name when talking to Six in the penultimate level.
What Could Have Been: According to Bungie, the original plan called for NOBLE Team to escape the glassing of New Alexandria by hijacking a Covenant scarab, then letting the player pilot it out of the city.
And the huge-scale battle cutscene at the beginning of Tip of the Spear? That might have been partially playable, judging from some cut content videos.
Concept art and demo videos exist for a "Warthog of the Seas", aka a speedboat with a turret.
Remember Sára Sorvad, the orphaned lady from the first level? She was going to accompany NOBLE Team as a science advisor.