A reboot, of sorts, is what Halo needs. First and foremost, it needs a creative director that is deeply/immensely passionate about it, its story, its gameplay and its legend. That person has to know what a good idea is, and must be able to lead management in such a way that teams maximise time, quality output, budget and ultimately efficiency.
It doesn't need a hard reboot, per say, but it definitely needs some time away from the gaming scene, so that whey it returns it feels fresh/novel. But it must also play fresh/novel. Obviously, you don't remove what already works, and Halo still has a fair bit going for it, that works extremely well - that's the gunplay, general gameplay and basic story (Super soldier saving mankind from crazy aliens). But it definitely needs more. By that I mean, while you're a super soldier, you never actually feel like one. Because your enemies are almost as large (or larger) and as strong as you, you never my feel powerful fighting them. At least, not nearly as powerful as how that power is showcased in movies and TV (if it's one thing the the Halo TV show nailed was showing off how powerful Spartans are).
With this, you have to be careful because while you want to portray the Chief as being a serious force of power, you don't want to turn the game into something else. You don't want it to feel like Infamous, Prototype or even Crysis. You want to retain the "feel" of Halo, but you'll need more than just the run and gun approach that Halo used for years. The introduction of the grappling hook was a great way of adding more dynamics to the gameplay, but I believe there's still a whole lot more that can be done. It will take some experimentation, but that's what time in the incubator is for.
Halo still has an extremely large, almost religious fanbase, and a lot of people are still rooting for it. People want to see Halo be great again, and with the right person or people in charge, it most definitely can be.