Here are a few ideas,
1st - the campaign
-please do not have the flood in halo 4, have it in halo 5,6
-same with the cov. or at least not with the chief
-bring back the halos, and Dyson sphere
-and have more forerunner stuff and/or them
2nd - the multiplayer/forge
-fix infection, glitches occur when trying to infect such as the 'betrayal' and not killing glitch (cutting shields and not health)
-very little bloom, gets annoying when you have to wait for it to go down
-customizable elites, halo 3 was good not Reach
-cR earnings for armour, NOT for ranking up, exp does that
3rd - the actual hardware (ie manuals)
-please have in depth manuals, not like CE:A, halo CE - 3 and Reach manuals were great
-limited/legendary editions?
that's all my thoughts
GO 343i