Two words, the Flood.
I mean Cortana is on one of the Halo rings, and who knows, some Flood spores were probably there for research like what Guilty Spark did on the first ring. I mean come on, a four way battle between humans, Covenant, Promethean, and Flood. That could rival the level in Halo 2 where we see a huge battle happening in the Arbitor level, and the one where Master Chief goes aboard High Charity.
I want amazing graphics and cool scenery
HamCon55 they did something like that in the opening level of H5, but I know what you mean about having ALL of the factions in it too. I think that’d be cool too, especially if you combine it with what I’m hoping to see, which are special appearances from iconic persona of past events (the Sangheili councilman from H3, Veronica Dare from ODST, Spartan Grey Team, etc.). They don’t have to be foundational roles, but big enough to make some sort of difference. That said, they’d have to ensure they have enough in-game exposition for newer people to know who tf they are, because not doing that was one of the ways H5’s storyline fell a bit short imo.
tl;dr – Huge final campaign (like the Earth/Arc campaign in Halo 3), and a reunion with a whole ton of Halo universe VIPs