I would like a COD Zombies style game mode, like a Flood Infeststion DLC, where a group of marines, ODSTs or Spartan IVs-IIs discover the secrets and origins of the Gravemind and the Flood.
It’s a little late for Halo 5 for those. But there are plenty of Halo 6 wishlist threads in General Discussion that you can use for future games.