Core gameplay mechanics are not good things to change. The reason Reach received so much hate from long time Halo fans was because it completely changed the way you have to play the game. The people for change say that Halo needs to change or it will be like Cod, putting out the same game every year. However, they fail to realize that Reach is the closest thing to Cod of any of the Halo games.
Change isn’t necessarily a bad thing. New gametypes like Firefight and Invasion were excellent additions to the matchmaking experience. New weapons and vehicles like the Grenade Launcher and the Falcon are great additions as well. But Reach completely changed the way Halo feels and plays. Armor Abilities destroy great small to mid sized maps since you can navigate them faster than you are supposed to. Bloom was a failure all around, Im pretty sure everyone can agree on that. The explosives in Reach are completely overpowered. The blast radius is incredibly large compared to Halo 3. All you have to do with nades or Rockets in Reach is aim in the general area and there is no way for the person to avoid death.
Halo 3 had arguably the best balance of any of the Halo games. All of the weapons had their uses and could be beaten given the right circumstances. The movement speed allowed for fast paced gameplay. The maps weren’t all over sized to account for things like sprint or jetpacks. These were the core aspects of Halo. This design should be improved upon. Not changed simply for the sake of changing like what was done with Reach.
TL;DR Halo shouldn’t change just because fanboys have something against Cod not changing. Halo should stick to it’s core gameplay mechanics and improve upon them.