Personally I’d like - despite the potential for it to go horribly wrong - to see a treatment of the First Crusade. Even sticking just with the Christian side you’ve got your sophisticated but overwhelmed Byzantines, your land-hungry Normans, quite land-hungry but also true-believer leaders like Raymond of Toulouse, and of course the rank and file, not to mention the People’s Crusade of visionaires and no-hopers. It’s a fascinating mix of realpolitik and genuine religious belief, there’s any number of political and military crises to act as a focus for this theme, and that’s before you get into the perspective of the Muslim elites and largely-but-not-entirely Muslim locals having to deal with this crisis.
Obviously, it could go badly wrong because the central tenet of the Crusade was that killing Muslims made god happy, so the challenge would be to present this ethos without endorsing it. But it could be done - Ridley Scott’s Kingdom of Heaven managed pretty well - and it’s an epic story to play with. And of course, the actual capture of Jerusalem and the formation of Crusader Kingdoms in the Near East is really just the start…