Halo Wars is starting to look like an amazing RTS game. Some of the recent screenshots reveal just how massive these battles can get. However, the game comes up short when it comes to factions. As of now, only the humans and covenant are playable and only the humans have a campaign, but I'm sure that Ensemble will make one for the covenant. What this game needs now is more races and I have the perfect couple: Forerunners and Flood. Neither of these factions have been playable in any Halo game, whereas in the Halo FPS series you can play as both humans and covenant thoughout the capaign and get a taste of their weapons, vehicles, and tactics. Anyone who has played Halo and payed attention to the story must be a little curious as to the history of the Forerunners and their conflict with the Flood. Neither the Flood nor Forerunenrs have been playable ever and with a new Halo game comes a new chance to introduce them for the first time.
Now most may argue that the Forerunners and over 100,000 years extinct and that the Flood haven't been encountered in this game's timeline. But I have the solution: add a second storyline. A completely different campaign about the fight between the Forerunners and the Flood, and the eventual self-destruction of the Forerunners. Perhaps in the current storyline the UNSC marines find an old Forerunner stucture and use it to hold off a covenant attack. After this fight, the story goes back 100,000 years to show how the Forerunners built it as fortress against the Flood and another battle takes place. Now this may seem repetitive, but not all the fights need to be in the same place (the Marines may continue north while the Forerunners push the Flood back into the south).
Having 2 stories would make it possible to add these 2 factions wihle keeping the Halo story intact. And with these 2 factions in the game, they could be used to fight the humans or covenant in multiplayer matches. Adding these 2 races, I believe, is the difference between a "pretty good" game and an "incredible" game.

If I can't get a horse to drink, I drown the d*** thing!