The reason the first halo games were so successful was because they made small improvements while still staying true to their roots. And the reason some halo were not successful was because either the improvements they added to the game did not help the game as much as they thought and a result hurt the game, or the game was not true to its roots.
The improvements made to Halo 4 were not as good as everyone thought they would be and in many ways hurt the game and did not deliver that true Halo experience that everyone loves.
There are a few things that Halo 4 is lacking that needs to be fixed in the next game, these things are necessary things that gave us the original halo feeling that we love so much.
The gameplay NEEDS to be slowed down. The simple fact is that instant respawn, sprint, and kill streaks have sped the game up to much that I get the same feeling that I do when I play Call of Duty. Even the Campaign seems rushed. In previous games each campaign mission was split into a number of stages, all of which made the game feel slower and less fast paced. In halo 4 you are on the edge of your seat the whole time and there is to much action to really understand what is going on in the story.
No more loadouts. Loadouts were a unnecessary feature in the game the served absolutely no purpose except to speed up the game even further. The multiplayer in Halo 1, 2, and 3 was fun because it was half about finding the guns that you like and really added some depth to the game. This also added to the slower paced gameplay which is a KEY feature in Halo multiplayer.
The return of Firefight . When people are tired of playing campaign missions but don't want to play multiplayer, firefight is the perfect to blow off some steam and have some fun. Spartan Ops tried to take the place of firefight but simply did not deliver the same experience that firefight delivered.
Armour Abilities needs to be removed. Armour abilities are what started unbalanced gameplay mechanics in Halo in the first place. In Halo Reach Bungie was trying to change their equipment idea into something cooler and more useful which resulted in a bunch of game mechanics that just made games unfair and frustrating to play. If anything bring back equipments. Even though they were not the most useful or cool looking, they sometimes came in handy and I think were a valuable edition to Halo 3's gameplay.
A Beta. A Beta is necessary so that we, the Halo fans who are buying your games can help identify problems with the game before the it is released.
I'm not saying that these things will make Halo 5 the best video game on the market. But they need to happen If you want the game to be remotely successful and it is a great place to start.