Hopefully, they'll update Infection to have the Flood. For example, picture this average game of Infection in Halo: Reach... A Spartan stands on top of the bunker, looking in all directions. Out of nowhere, an Alpha Zombie jumps out and slaughters him. Then, a small invincible Infection form jumps down from out of the map and runs over to the Spartan's body. It cuts a gash in the armor's chestplate, and jumps in. The body worms around on the ground. A Flood arm with tentacles explodes out of the spartan's right arm armor. The chestplate bulges out, and Flood tentacles poke out the gap in the chest. Various other parts of the armor break up and tentacles poke out. Then, the invincible infected Spartan stands up, runs over to the Zombie spawn point, at which point the player who got infected regains control of the newly infected Spartan, who then loses his invincibility. The game then goes on normally. The Zombie respawn points could be holes in the ground filled with Flood crap that you can't enter, but when the Zombies are killed, they spawn in there and jump out. There could be Zombie Custom Powerups that mutate your Spartan into a Tank form, a Ranged form, or a Stalker form. Now wouldn't that be epic?
There IS a forerunner structure on the planet Reach. Menachite, I believe its called. You can read about it some on HaloWiki or whatever, or in The Ghosts of Onyx and one of the other books as well I think.
Wasn't that the reason the Covenant were invading with ground forces instead of straight up glassing Reach from orbit? The forerunner artifact was their primary objective, and they're knocking out MAC generators too so they can move their ships in closer eventually. I'm thinking the game begins with your character doing some "lone wolf crap" like they mention in the cinematic. That's probably him defending a MAC generator. After that mission, he joins up with the squad, we see the already shown cinematic, and we're off to first find out what the covenant are after, then stop the covenant from getting whatever it is they're after, then when we thwart them they start glassing and we've got one of Halo's climactic warthog escape sequences as our character rushes off to bunker up with Hasley and the other survivors. Legendary ending of course shows Johnson Master Chief and Cortana rescuing our character and Hasley's team.