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.