There are several suggestions that frequently crop up on this forum and have been discussed thoroughly in the past. For everyone's convenience, I've compiled a list of as many topics as I and the other members could think of and added related links to each of them, so you can just read up on whatever you're thinking of posting about instead of making a new post, identical to all the earlier ones. If you have a new take on the subject, you can always join in the discussion, but if your idea isn't on this list or you don't want to bump a thread that's several months old (Most of these are pretty recent, though), then go ahead and start a new one. As long as you read this post.
Shield bash feature
Dual wielding weapons
Formations for troops
Tabards and heraldry
Gore, dismemberment, decapitation etc.
Crafting
Roads
Hunting and fishing
Captains, lieutenants and other NPC characters
Enemy heroes
Camping your troops
Sieges
Magic, magical items and magical creatures
Multiplayer
A new tournament system
Owning land
Sprinting, dodging, rolling, stamina etc.
Storyline-related issues
Picking up items on the battlefield
Ability to ready your pike against horsemen
Custom battles
Ability to knock horsemen out of the saddle
More armor slots
Permanent death
Excessive money
Jettisoning cargo while travelling
A health display for horses
Firearms
Ransoming your prisoners
Battle replays
Powerful attacks breaking parries
Female armor
New weapons
New armor
New horses
Crouching and improving archery
Power draw implementation and skills dealing with archery
Axe fighting and inquiries on blocking and deflection
Ragdoll Physics and its many possible uses
Voices of characters
Shield bash feature
Dual wielding weapons
Formations for troops
Tabards and heraldry
Gore, dismemberment, decapitation etc.
Crafting
Roads
Hunting and fishing
Captains, lieutenants and other NPC characters
Enemy heroes
Camping your troops
Sieges
Magic, magical items and magical creatures
Multiplayer
A new tournament system
Owning land
Sprinting, dodging, rolling, stamina etc.
Storyline-related issues
Picking up items on the battlefield
Ability to ready your pike against horsemen
Custom battles
Ability to knock horsemen out of the saddle
More armor slots
Permanent death
Excessive money
Jettisoning cargo while travelling
A health display for horses
Firearms
Ransoming your prisoners
Battle replays
Powerful attacks breaking parries
Female armor
New weapons
New armor
New horses
Crouching and improving archery
Power draw implementation and skills dealing with archery
Axe fighting and inquiries on blocking and deflection
Ragdoll Physics and its many possible uses
Voices of characters