Make maps about three times larger and have "capture points" throughout the map(initial spawn, a center spawn and two in between for a total of 5). Either side can capture any of the points. If one side captures all points, they route the enemy. To capture a point you need a minimum of 10% of your team on the point, to stop a capture you only need one person. This goes for battle, siege, village, castle, or town. Capture based on proximity, not like pressing some arbitrary key at the right time between enemy spawns to lower the flag. Having 5 times the amount of players on a capture point than what the owning faction has, should keep the owner's faction from spawning there.
Wave spawns rather than spawn time relying on death count(besides, this seems to cause excessive lag). Either place a battalion flag on top of a capture point or have the battalion leader set that point as the wave respawn point to have his battalion spawn there(just press 'f' while standing over it). 35 second wave respawn timer base, -5~ seconds per capture point your team controls. So starting timer is 30, wave respawn timer with 4 of 5 captured points is 15 seconds.
Retreat mechanics should be reworked so that players have to actually run to the edge of the map to retreat. Add retreat requirements so that players can't simply retreat at 10 tickets to keep from getting teleported. If you are overwhelmed in troop count, you should not be able to retreat easily in a field battle. If your faction is severely losing, retreat should be harder to initiate. Possibly have the retreat requirement look at the K:D ratio of the previous 5 minutes of the battle and if it's too low (1:2), you are then unable to retreat. If not enough people have died(retreating side isn't getting pressured, for example) retreat becomes possible.
Upon retreat initiation, whatever percent of players successfully retreated(rather than getting chased down and killed) would be the percent of troops left alive after the battle end. Finally, battle victors should have a chance to capture defeated commanders who would stay captured until freed or exchanged.
Drastically increase upkeep so that you see army sizes much closer to what single player has. Make reinforcement work the same way it works in single player. Allied army sets a guy to follow, when he enters into a battle anyone on follow will continue to move until he is within reinforce range, then his party will "reinforce" and the troops/equipment will be added to the what the final battle has. Players not already on follow can simply move their character until they are within reinforcement range, then chose to reinforce to the same effect.
Remove (could rework it somehow, maybe) the raid system.
Allow fief owners to delegate others with fief privileges. (Garrison manager?)
Have catapults cause permanent damage to structures unless a fief manager repairs them.
Battering rams.
Ballistae.
Fief upgrades: Stronger walls, upgraded gatehouse, moat, iron as opposed to wooden doors, pitch drops.