Warzone had some great ideas, but there are somethings that would need a change if it returned. Well, in my opinion any way. One of those things is less uniform bases. By that, I mean the bases should not be the same on every map. Some maps had the mid base as a forerunner structure, but in many it was just the white round building. This should not return. Each map should have unique bases, whether that be a covenant structure, abandoned building, Forerunner structure, etc. It just shouldn't be the same garage every time. Same goes for the two team bases.
Balancing is also important. I get that the all out mayhem of warzone can be attractive to some, but there needs to be some balance with certain things. If something is absolutely broken, then it should be limited to one spawn. There's also the issue of rockets, and spartan lasers when it comes to vehicles. The Warthog in Halo 5 is essentially useless, as with much of the vehicle combat that doesn't involve the Banshee, or the Scorpion. Vehicles and weapons should not be made obsolete. How they would go about doing this, I've no clue. But it wasn't just an issue with Warzone, but the whole of Halo 5.
Greater diversity of bosses, and events would be a much needed improvement. Perhaps including remade past bosses such as Tartarus, Prophet of Regret, a Monitor, etc. could help, but they would need entirely new move sets and functionality. So, essentially, might as well just make new bosses. I get tired of fighting the Warden Eternal the two hundredth time but this time in red. Bringing in characters like The Didact, The Gravemind, Tartarus, Atriox, etc. would really play to fan service, as well as provide much better boss fights.
Bigger maps, more players, better A.I. and more importance placed on team A.I. There should be ODST's, along side Marines. The A.I. should at times assist in pushes, or at the very least get inside of vehicles. Just having three Marines sitting inside a small base with their thumbs up their -Yoink- doesn't really add the experience. Larger maps, that have more diverse play areas, such as buildings, close quarter combat, different elevations and levels (more than just a hill, and not a hill). More forested areas, or rocky crags, or interiors. Just more than what is essentially a warthog track with three bases.
More game modes. An Elite versus Spartan mode would be excellent assuming they bring back playable elites. But that's only one idea.
And last, and certainly least (a small nitpick), don't make the Marines look like they're wearing nerf armor. Give them colour stripes over the green armor, or something, just don't make them look like nerf warriors.
Not sure if all of these will be agreed with, but I think they're important. If any one thinks I'm wrong, just call me stupid for my opinion or whatever, as is customary of the internet.