I don't think we'll get more. :(

Now that I've thought about it, though, I'd love Halo Wars to be spun off into its own series that focuses on the Human-Covenant war and the immediate aftermath. No Prometheans. No Forerunner. No save-the-galaxy plots. Just the interaction between the UNSC, Innies, Heretics, Covenant, and Covenant Separatists.

And even if Halo Wars is its own series, there's no need for the gameplay to be a constant. Some games could be an updated Halo Wars, others would be more tactical games like XCOM or Men of War.