These are my suggestions, I'm sure that variants to these suggestions have been made before. I'll put forward Arguments for why they would be good. Keep in mind that none of them are motivated by any sense of overpowered-ness. They are mostly interested in making gameplay more 'interesting' [mostly aesthetic improvements]
1) The ability of musket wielding infantry units to fix and remove bayonets.
Muskets without bayonets could have any one of the following advantages, [or none at all, as musicians and banner carriers serve the same function]
a. moderate increases in reloading time
b. moderate increases in accuracy
c. allows for the musket to be used like a club instead of a spear, while a 'club' melee-based musket would lack the high damage offered by a bayonet stab, it would allow the user in melee to swing his musket side to side rather than simply stabbing and overhead swinging. Club swings would naturally need to be slower than regular stabbing for balance purposes, and the blunt damage would be less than that of a sword swing.
Fixing Bayonets could only be performed whilst standing still.
On top of this suggestion, you could make the Bayonet into an actual independent weapon [which 'appears' and 'disappears' in the players inventory depending on whether it is fixed or not] The bayonet could be used like a dagger. I'm not certain of the historical accuracy of using a triangular bayonet like a dagger, i've only seen it used that way in the Sharpe Series, and on historical accuracy i'll take with a grain of salt.
However, it would appear realistic for such a feature to appear for British Rifleman, since their bayonets WERE sword bayonets.
Also keep in mind that the triangular bayonet, if it were to be used as a dagger [which, ironically, the first bayonets were] i'm not suggesting that players be able to block with them, though it would make sense to block with the bayonet sword. ALSO, i am not advocating that rifleman be anywhere near as good or effective with bayonet swords as cavalrymen or officer.
2) March Movement
This feature is more for the sake of line battles than for practical fighting. Basically a March feature would allow all infantry units to walk at an exact pace, a pace that could be matched by all other infantrymen [pipers, drummers, color sergeants, etc.] This movement pace would not, unlike regular running, increase the speed of the player depending upon how long they've been jogging or whether or not they are moving up/down hills. It would also allow musicians to play whilst moving.
Allowing this feature would open the possibility of line battle regiments from closing in on one another whilst in line, as opposed to having to approach each other in column to maintain cohesion and only exiting line whilst at the last moment.
3) Charge Bayonets
Referring to the ability of a bayonet weilding infantryman to point the bayonet forwards from the hip but not in a motion that indicates either a stab or aiming.
To see what i'm talking about, watch this clip at 4:10
http://www.youtube.com/watch?v=hjWMEGQOcg4
The idea would be similar to that of a couched lance. I have heard proposals on this thread elsewhere suggesting a bayonet positioning that would deal damage to enemies that ran into it, in a passive manner. I'm not advocating this persay.
What would the use of a Charged bayonet position be though, besides the aesthetic appeal?
I am thinking perhaps it would allow soldiers to fire their muskets without having to enter and exit an animation, that is to say, in charged bayonet position, you can fire a musket immediately once the animation has been entered into, without having to alert your enemy when you are going to fire. Naturally, firing from the hip has an accuracy penalty, and so this sort of firing mode would only be practical at close range. [I'm aware that muskets are already extremely inaccurate and so this seems pointless, i'll talk about my thoughts on musket accuracy elsewhere] However, consider that the ability to fire a musket immediately without having to enter into a firing mode is a major asset.
The accuracy penalty for firing whilst marching [this could only be done whilst marching] would be lower whilst in charged bayonet mode than whilst aiming regularly but running/jogging
Admittedly i'm not entirely sure about just how useful this ability would be in actual combat, besides the fact that it would look intimidating. But it isn't like MM hasn't included things with little actual practical value that people find cool to use. [musicians and banners and such] You COULD incorporate a couched lance feature that would deal damage to enemies who carelessly ran directly into the bayonet, but you would need to make sure that this damage did not work for running into the side of the bayonet. [i.e ONLY worked for the bayonet point and nothing else] I personally think this would make it too overpowered.
You could, also make it so that horses could not run directly into a line infantry unit who had his bayonet charged. [the horse will rear if it tries, instead of taking damage, I consider this fair and balanced since the horse would not actually impale itself on the bayonet, but would stop, allowing the infantryman to stab the horse once he exits out of the charged bayonet mode.
Under such a situation, an individual line infantry man could only protect himself from cavalry from his direct front. A horseman could still kill him by attacking him along the side, flanks, or rear. However, a group of multiple infantryman could now form a much more formidable square [in reality, a large circle rather than a true square]
One issue that would need to be dealt with is preventing people from going into charged bayonet mode and pivoting around in order to effortlessly defend against any cavalry. I'm not sure if you can code limitations into the pivoting speed of a player since the rotation speed is typically determined by the settings the player has.
4) Officer Spyglass
I'm not sure if anyone has ever suggested this, but it might make sense giving the officer class [since he has 1 slot remaining after getting a sword, pistol, and cartriges] a spyglass, for scouting use. I'm not sure if it is possible to code this, but basically the spyglass, when the officer hits the standard attack button, would allow the officer to get a zoom-in view ahead of him with a degree of vision much longer than that of a standard shift-zoom.
I'm also in favor of removing the feature for common infantryman and carbine using units to use the zoom feature for their muskets, and reserving this feature for light infantry only.
5) Musket accuracy
First let me preface: I understand why the present level of accuracy for muskets is utilized. [Or at least I think I do] I'm certain that the developers of MM are aware that someone trained with a regular musket could fairly routinely hit a man-sized target at 100 yards or more, [massed volleys themselves were less accurate for several reasons I won't mention here] And I am fairly certain that the level of accuracy of muskets is designed to accomplish the dual goals of 1. Making reloading times of less than realistic length so as to not bore the player to death with reloading 2. Not make muskets so accurate as to render melee engagements impractical. So unrealistically low accuracy and unrealistically fast reloading would naturally be preferred.
So I understand why advocating realistically accurate [Like the level of accuracy present in WFAS] muskets and realistically slow-firing muskets are not preferred.
HOWEVER, I ***think*** I may have come up with a few ways that could allow for muskets to be more close to their actual level of accuracy without turning every infantryman into a sharp shooter. Any one, or any combination of the below solutions could be implemented, but because I am not familiar with the coding of MB I am not certain if any of these things are possible.
a. Slow Closing reticule; accuracy versus ROF tradeoff.
Players who present their muskets and hold them steady for long periods of time will gradually see their reticulated close to an accuracy that would allow them to hit targets at a longer than usual range. Moving the Reticule quickly from one area from another will return the musket to the default 'inaccurate' state, but moving it slowly will have less of a degrading effect, and keeping is perfectly steady will maximize accuracy.
From what I've read and seen, one of the many reasons muskets were perceived by soldiers and officers as so inaccurate [besides weapon maintenance and the training of the soldiers] was the preference by officers for loading drills over target practice.
This means that a regiment can increase their accuracy by holding their fire, HOWEVER, the added accuracy takes a long enough time that, depending upon the range that the enemy is from you, firing off many shots with less accuracy is better than the alternative. [As is presently the case with muskets and rifles]
Rifles, however, still have an advantage to muskets in the sense that a rifleman needs less time to properly aim the weapon to get the same desired accuracy.
As an addendum, you could also permit enhanced accuracy granted by units that go into a kneeled position, this is also more realistic, however I am nervous about this particular suggestion, since crouched players already have the benefit of less profile. The key in any game is to have trade-offs, whereas with standing still for a long period of time to take a shot increases your profile, especially against cavalrymen.
b. Gunsway
You can [alternatively, either or] reduce the size of the default reticule for aiming a musket, but have that reticule bob back and forth slightly like most of today's FPS shooters do. If you time your shots properly you can land more accurate shots with a musket, but it requires more skill than with having a more steadily aimed rifle. [Makes sense because larger guns, if they are not mounted to anything, will have more sway] The gunsway could roughly emulate the range of the current musket accuracy [that is, the reticule sways within the space of the old reticule] But now shots are much less random in where they land.
As i mentioned at the top of this, it's true that muskets were not rifles, and bullets did bounce out of the muzzle, but the inaccuracy of a musket was *not* as bad as many think, especially when aimed, and this will partially emulate this difference.
c. Kick of the musket
This proposal is a bit of a stretch... but... w/e
I recall reading that british soldiers at
close range were taught to aim their musket at the feet of enemies, because the kick of the gun would cause a recoil that would send the bullet flying upwards. But I'm not sure how entirely true this is. However, if it is true, you could try to implement it in the game. Just as it is more difficult to hit a target with a weapon that requires you to aim very high [like a bow] Ceteris paribus, than a weapon that requires you to aim straight, it is probably more difficult to aim a weapon that requires you to aim lower than usual. [At someone's feet for example] You could increase the actual accuracy of the weapon, but implement this feature so as to require that players shooting muskets take more care with their shots, particularly at close range.
d. Simply remove the reticule
The musketeer would now have to aim without the aid of a reticule, even though his musket is more accurate than it was with the reticule. Players would have to get used to targeting enemies in third or first person, so players with more experience in targeting would be substantially better shooters than those without, again, this proposal is both balanced and realistic.
What all of these suggestions have in common is that the substitute the game-established inaccuracy of muskets by coded-randomness of the bullet with natural inaccuracy caused by the inadequate skill of the shooter. Such a change will allow for a greater diversity of skill among st players in terms of shooting accuracy that more closely resembles the greater diversity of skill among players in terms of melee abilities. [I.e. in the present system the difference in shooting skill between players is much narrower than in melee, especially in line battles]
7) Replace the paper cartridge model something more easily visible, like a cartridge pouch, or a powder horn [in the case of jaegers and green jackets]

Allow dragoons to reload their carbines while on horse, provided the horse is completely stationary.
This proposal has been made before, I am simply here to endorse it. The fear of mounted reloading stems from the fear of dragoons becoming what horse archers are in regular MB, extremely annoying. However a stationary horse is one of the easiest and most susceptible of all targets, more so than a dismounted dragoon, and so making reloading while stationary but mounted seems acceptable.
Admittedly i am also not sure if this can be coded.
9) Make the surrender animation more legit
The surrender animation occurs immediately when a player has no weapons equipped. Perhaps consider doing something similar to what AFI mod did and have it function as the 'block' animation for melee, and freezing your player whilst you hold block. Alternatively, you could have it function as a secondary weapon function [i.e. go into fists and then press X to surrender]
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I have other proposals, but I'll leave with these for now.