I agree there comes a certain tipping point that nothing is challenging, your kingdom just steamrolls the other factions to the point it is laughable. There needs to be a late game challenge, kind of like what Medieval 2 had with the Mongol invasion (epic gaming times). I would love to see a powerful Nord invasion like what happened in and is supported in lore with Gundig's invasion. This is something that could really change the dynamics of the player being comfy and having to worry about large Nord invasions and fleets wrecking havoc with port cities. There is not a more terrifying enemy that can be added to the game.
This would be a good time to being on the very requested feature of naval combat and travel with lots of great features like the following:
-Ship travel around the map
-Ship ownership (Warships, merchant ships, etc)
-Naval combat
-Port amphibious attacks from warships
-Merchant fleets and ship trading via ports
-Piracy and ship raiding
-New factions like the Merchants guild, Nords, and various pirate rebels like Cilician pirates and Corsairs
Also there needs to be an Imperial capital that is fortified like Constantinople and it would be the mother of all siege battle challenges. It would be the hardest settlement to capture and would be requisite to gain the tile of Emperor or something. You would have to bring your very best as the muderholes and choke points would be brutal. Where you have to cross a bridge under very well placed defenses kind of like in the image below.