This is listed by suggestions: 
1. Add a locus (from halo wars) as a covenant mech. 
2. Make the flood come back in the last mission of halo 5. 
3. Change the halo setting of halo 5 to erde-tyrene. 
4. Put the didact's war sphinx in halo 5 or 6's campaign. 
5. Make 2 war sphinx combine later in halo 6 after we have first seen and destroyed ateast 1 war sphinx.(see halo wiki for details) 
6. Infect the didact with the flood in halo 6 giving the flood control of the prometheans, ironically a force meant to stop the flood. 
7. make the humans and the flood work together to defeat the flood and the prometheans... 
8. Make a new gravemind in halo 6. 
9. Put a hornet or falcon to forge. 
10. Add a wider variety of halo gametypes. 
11. Make the flood mode more customizable... mainly infected traits and loadouts + no waypoint for last man standing. 
12. Allow the ability to turn off sprint. 
13. make the jet pack stronger... not just a useless jump boost 
14. Add more customization options to custom games. 
15.In the future halo's- make more arena style maps 
16. Bring back a skill ranking system like in halo 3 or 2. 
17. Make casual part of multiplayer and a skill based part.  
18. Put atleast 1 scarab between halo 5 and 6 
19. MAKE A GAME EXTENSION ALLOWING FIREFIGHT WITH THE FLOOD, PROMETHEANS, COVENANT. I SERIOUSLY WANT FLOOD IN FIREFIGHT! (Keep Spartan ops but add this in as a game extension similar to halo 3 odst.)(u have to buy the firefight disc) 
  
Feel free to make your own list on the forum. I want to see what everyone else thinks.