I think a Baltic crusade mod would be cool and it could re use a lot of content of the original. Sure I'd also love to see something set in the viking age or the 30 years war but why base it on the game at all if you have to change everything anyway. The Song of ice and fire universe (Game of Thrones) doesn't face this problem as parts of it are basically described the same as the game period. There actually are some advantages to fantasy, there's more room for player agency and you just have to worry about not contradicting a very limited amount of lore as oppose to the unlimited amounts of sources in real live. Also there we all have a distance to Westeros that we don't have to real live. Fantasy mods don't have to worry about being biased. But I don't think the Lord of the Rings lore is very useful for creating a believable story or world even if the movies had some very cool armor.
If some people were to put in the effort of doing a historical mod/total conversion I think the rule of Přemysl Otakar II. and his conflict with the habsburgs could be fun. But it's also not exactly the same time frame and might be way to much effort so just an other idea.
Edit: I forgot the most important, awesome and obvious mod idea, Mongol invasion!