Modder William Faure has one answer, with a mod that takes Enemy Territory’s maps and connects them together to form a story-driven campaign. There’s a trailer for it above, featuring a rad metal version of Für Elise and a whole lot of Nazi-shooting action. A co-op version of the mod will follow on November 30, and is currently in testing.To play it you’ll need to have Return to Castle Wolfenstein installed as well as one of its overhaul mods (iortcw, RealRTCW, RTCW4A, RTCW-Touch, vitaRTCW, or RTCWQuest). The Enemy Territory singleplayer mod is available on ModDB and the Steam Workshop.
If you’re interested in the saga of how this foundational FPS series was passed from developer to developer over the years, here’s the history of Wolfenstein.