I think that Halo 6 should have more vehicle based levels especially air type. For instance, Halo: Reach’s New Alexandria with the Falcon, Halo 3’s The Ark and 3 ODST’s Kizingo Blvd with the Scorpion. I also think there should be more UNSC involvement, like having marines fight with and maybe spartans also.
There’s a bunch of active wishlist threads you can use. 