I don't know if you've ever played WoW, but they have a system called Addons. These are mostly legal additions to the game made by the community and other programmers. One popular addon library is Curse, which allows free downloads of any addon on their site. An example of an addon is Bartender. This addon allows you to freely move ANY bar on your in-game screen. Say you wanted to put all of your action bars on the to of the screen, instead of the bottom. You could easily do this in a few clicks. Another addon is Recount. This addon allows you to track stats of everyone in your group. For example, you could track total damage to see who isn't carrying their weight. One last addon is Atlas. This is by far one of the best addons. It allows you to view every boss, in every dungeon(strike) and every raid. Then it even shows all possible loot from those bosses, along with drop percentages for each piece of loot. If Desyiny could utilize this addon idea just like their website custom map downloader for halo 3, I think it could allow for some awesome addons. Maybe they could call them HUD editors.