While those are great ideas, I honestly don't think that 343 would have the resources to do all of that AND work on Halo 6. Which should be priority right now.
They should make Halo 6 Feature complete.
Fix the BR model and get rid of ADS
Bring back Warzone for the Whales.
Firefight for the ODST nostalgia
the DLC and Maps should be free because Warzone.
Give the option for a one time premium payment to just unlock all cosmetics, along with people who want the Req system.
Continue the Competitive Focus, Better maps preferred. (especially if they tie into H3A at all)
PC release for H3A and H6
Make the Jackals look like Jackals
Multi Species Multiplayer E.g. Spartans, Elites, Maybe Brutes.
and a gun that shoots Cuban Sandwiches