I think it would be a great step forward to reduce the area of conflict from Calradia to simply a border region between, say, the Khergits, Swadians, and Sarranids. This would free up some unit balancing and equipment design time from your developers to instead implement game-play improvements like...
* First off, please don't be afraid to make the graphics a little under the status-quo in order to focus on game-play first.
* No world map, walk / ride between castles/towns in randomly generated plains, forests, deserts.
* Smaller scope of battles would allow more accurate depictions of skirmishes... not battles. Since the player army is typically around 75-150 units strong, it makes more sense to focus on skirmishes over battles - the smaller theatre of war allows you to take that limitation and roll with it. Focusing on skirmishes would also translate better into multiplayer if you decide to make an objective based mode with more flavor than capture the flag (Stealing cattle, looting a village, etc.)
* Focus on the player's roll as leading a mercenary warband, bandit warband, or lesser lord for a kingdom. Don't open the can of worms that is 'player kingdoms' at all... leave that to modders.
* Have multiplayer 'macro game' where 'team captain battle' style skirmishes are fought on servers to capture land/income. This income can then be used to improve settlements, upgrade troops/weapons. This would also provide an incentive to raid villages. Borrow as many elements from the old MMO, Shadowbane as realistically possible, here.
* If your engine isn't already locked in, strongly consider destructible environments, dismemberment, and physics that cause knock-backs and trampling from coming in contact with a galloping horse, or a warhammer. The combat in the Mount&Blade series is the huge selling point, but simply adding a little more dynamism to the results of such powerful forces bashing into either would go a long way towards attracting a wider audience.