[@EliteCommander] Personally the only things I can think of that would warrant a joint operation might be a Covenant splinter faction (perhaps The Banished) making some trouble in the area, a Flood outbreak (which could lend to some horror elements), or perhaps even some guerrilla warfare against The Created.

Only somewhat related, but [url=https://www.halopedia.org/images/0/09/TheDuel_Elite_Concept.png]this[/url] concept art would make for an awesome Elite design.