To specifically make the LATE game (kingdom phase) more fun:
* Bring back from Warband
a way for the player to easily contact people and make vassalage, mercenary or marriage offers during the kingdom phase; e.g. kingdom court which people visit, or messenger system. This will take out frustration of having to chase people down when you have more important things to do.
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More fun diversions from the endless loop of sieges, menus and field battles in the lategame. Being attacked by assassins, being challenged to duels, hosting and attending feasts, or quests that are appropriate for a king to do and have a suitable reward; things like these will make lategame less repetitive and more fun.
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Make battles have Good Tactics
. Fixing the biggest gameplay issues of cavalry not being able to land melee attacks making them underpowered, ranged units doing too much damage to armour making them overpowered, and spears and pikes not working properly, will remove the most obvious tactic that works against everything of spamming archers/horse archers and actually make us need to think about different tactics for different situations, thus making battles more challenging and fun! Morale should be more relevant too.
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Change the calculations for war, peace and tribute to be less frustrating so that we get occasional periods of peacetime and so that we actually can get tribute or peace out of a faction when we're beating them.
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Make autoresolve stop randomly killing high tier troops that would never die in the actual battle so that we can skip curbstomp battles and not get punished for doing so.
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Make AI kingdoms that have had no territory for 10 days stop existing (all their clans defect or go into exile) so they don't attack us forever after being defeated. And make them pay for their mercenaries!
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Add a cool down to the AI calling war/peace votes so we don't have to burn influence rejecting the same dumb vote over and over.
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Garrisons and Nobles should be more willing to negotiate or surrender so we don't have to actually sit through a whole siege to capture a castle which only has 30 men in the garrison when you have 1000.
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Engineering skill should speed up siege camp and siege engine construction so we spend less time waiting for 100 siege camps to win the game.
Do these things and lategame will be fun, rather than its "I want to jam pencils into my eyeballs" repetitive frustrating current state.