What Halo 5 needs to do is fix the issues created in Halo 4, but also to move the series forward in some way. This can't be a gimmicky attempt to move things forward, but needs to actually take some elements of the sandbox or some core mechanics and change them a bit without actually breaking them. Weapons need to be balanced better, loadout options and armor abilities need to be refined. 343 just needs to pay attention to some of the wider implications of the smallest changes they make. The game needs to actually support competitive play out of the box also. The multiplayer sandbox has been the worst part of Halo 4, so that is what needs to be reevaluated. This isn't as tall of an order as it may sound, as making the game competitive again is as simple as removing some of the broken options added in Halo 4. walking the line between allowing players to customize loadouts and whatnot without breaking the sandbox wouldn't be too difficult.
Beyond that, I'm not too particular. Halo 4's forge has a lot of bugs which could be ironed out, and the design of the canvases could be better, Seeing the return of one-sided game modes and more customization options in game modes is very necessary. Halo 4's campaign was a pretty big disappointment to me as a long-time Halo campaign fan, both in terms of story and level design. In contrast to the multiplayer sandbox's flaws, I believe this to be something which 343 just needs to keep doing to get better at, as opposed to needing to reevaluate the entire design process. On the art and sound design side of things, I really think they need to just continue in the same direction they went with Halo 4.