This is a very great game, but it seems like it's missing a few things.
1. The campaign should either have more levels or longer levels, many people think that; I think that too. There should also be a Covenant campaign for Covenant lovers. (Like me; because I'm a fan of Grunts!)
2. The other thing is that it should be able to have is something that is customizable because last time I played, it didn't have anything customizable, other than the Flaming Warthog and the Honor Guard Wraith. In the other Halo games, there were customizable characters. Maybe Robot Entertainment (the new company owning Halo Wars by former Ensemble Studios members since the company was shut down by Microsoft) could add updates for that on the Xbox Live Marketplace.
3. Even though I think that the skirmish heroes and abilities are very good, maybe Anders's "half price for unit upgrades" isn't very good because it doesn't reduce all the prices. I think the Flood should be playable too.
Other than those and a few other things, it's a fun game for RTS players, just needs a few improvements.