There are many small things that could make Halo 5 the greatest game ever. 
1- More UNSC weapons, different assault rifles and bring back the smg 
2- Customize guns, more weapon skins and like ODST make the guns have silencers and scopes 
3- Play as Arbiter like Halo 2 
4- More character customization, hight, weight, right or left handedness 
5- More armor skins that can go on all armor 
6- Bigger team battles, 10v10 players for multiple game tipes (Dominion) 
7- Make a mix between Spartain Ops and Firefight but bring firefight back. 
8- For firefight make it matchmaking with 10 allies or so with more enemies, and boss fights 
9- For fun bring back the Katana and make it behave like the energy sword 
10- Lastly longer campaign