My suggestions are for multiplayer. I would love to see:
MP Campaign match mode - Players would command or join factions in an online campaign match, they would ally, trade, build, loot and campaign across the map. Various winning conditions could apply. Buildings, armour and abilities could be upgraded as a faction progresses during a match.
MP Campaign world mode - Set in a persistant world server (maybe even base the world on real world locations?), players would join a faction village/town then work, train, drink, explore, trade, quest with their fellow players. When a neigboring faction orders a military action against your faction via a vote or by commanders order, you will be called to arms and will spawn in the appropriate battle to fight it out. Castles and units will be upgradable, day/nightime cyles for added strategy, daring raids or assasinations, NCP interactions, etc. Players would start as humble peasants, used as front line fodder in battles, then progress to knights or even the regional hero!
Resources - based on the campaign MP games, it would be nice to see some resources, these could be simple as regions having mines, etc, located at various villages, the faction that holds that village of course gains that resource for their economy. Teams would need to defend their resources to ensure they stay on top (trading, weapon & armour production etc).
AI upgrade - I would love to see the bots movement/waypoint AI improved, as when playing 1 v 1 with my friend in a seige, the attacking bots are pretty useless. Please see next point for a possible fix.
Destructable walls - I realise that this could be tough to implement if the player is able to chose which part of the wall he wishes to smash, however if the catapult/seige weapon is static and set for the player and the wall has only one "breach point" of destructable wall, then it should be fairly do-able. This would mean that players/bots could smash a castle wall at a section and then pour into the town/castle in a "kingdom of heaven" style confrontation. This would also help the bots assist with a seige, rather than programming more AI.
More & better maps - Many of the maps are a little too elevated, it would be nice to see some maps based on real places, and in general lower elevated hills. "Field by a river" seems like quite a realistic place, but many others do not.
Spawning - The issue of spawning randomly should be fixed.
I have so many suggestions for this game, but here are my primary ones.
