This is the only mod I've played that has made me create an forum account and post. A lot of potential is to be found here. I have a few suggestions but the first I wanted to point out: I found a very useful and interactive map of the USA during 1866.
Http://www.davidrumsey.com/luna/servlet/detail/RUMSEY~8~1~4225~420055:Map-of-the-United-States-and-Territ showcases a lot of details but as far as this mod is concerned, locating Texas is the biggest thing to look at. Unfortunately it only shows the border of Mexico and no details for it, sadly. The interactive map also does not show terrain as far as color wise. This map is would be most useful for the 1866, I believe as it shows towns and forts, locations and names. Simply zoom in using your mouse wheel and scroll until you find Texas. Seeing as how 1866 seems to deal mostly with Texas and Mexico along the Rio Grande, this map should prove awe inspiring. I'll see if I can find a likewise map for Mexico as well and post it if needed or wanted. This next map,
http://lib.utexas.edu/maps/historical/tex_fedforts_1866.jpg, is a much more simpler map but shows Indian war trails/warpaths through Texas and into Mexico.
Now for some more or less forms of suggestions.
Towns could be governed by Mayors, regions by Governors. Forts should be ruled by military commanders.
The faction leader should be the current ruler such as the president and such for major nations, or a warchief, etc. Though the president of the USA would never directly lead the nation in Texas, it could be delegated to the regional governor or commander?
You can volunteer for military service but you are limited to a term of years, not months. If you join an army there should be army ranks and more rewards/responsibility as you get promoted. Obviously you can't just join as a regular soldier, what fun would that be? They would only accept proven leaders of men into 'temporary?' officer ranks. You would have a set pay per month that rises with every promotion or lowers with every demotion. Maybe you will be given the task to besiege said fort and take it over. If you have a good relationship, skills, and the right rank you might be able to assume commander-ship of said fort.
Army wise. No man ever lead a massive army of a hundred troops or more, not unless he was an officer in a army or a mighty influential and powerful person. I think all low tier troops should be recruitable (with exceptions) for free if you belong to a Nation/army. If you are a Captain for the Union then you should be able to get some initial troops for free and given a station to replenish supplies, etc. Of course you would have to take your monthly payments, etc to maybe pay for higher troop tiers but still. No commander in the USA army had to pay to recruit his own troops. Now, if a commander wanted to equip his troops with better equipment and the like then just maybe it came out of his own pocket? Maybe instead of paying for higher troop tiers you would in fact have to pay for better equipped soldiers? Union Regular (Basic - Free), Union Regular (Silver Star eq - $1 per soldier), Union Regular (Gold Star eq - $10 per soldier), etc? Also if you had a few infantry and wanted to make them into a calvary unit, it would cost you a pretty penny to give all your troops horses, thus turning them from Infantry Regulars to Calvary, etc?
Of course some things could and could not be done in M&B, but these are just suggestions. If you did not belong to an Nation/Army then everything would be your cost. If you was an outlaw or bounty hunter or posse, etc, it would be up to you to pay to get the soldiers, train them, equip them, etc. Of course it is very unrealistic to see a outlaw gang running around 100 strong, etc. Maybe a script where at the end of every battle loot and money won is auto divided between you and your followers? The bigger your following the less loot and money you get at the end of the raid, fight, etc?
Someone has mentioned this but being able to own property, esp a ranch, would be awesome. Maybe you wanted to give up fighting and live the quiet life? Just served two or more years in the army and retired. Now you want to go back to your homestead and just raise livestock and live the good life? Why not, you earned it.
Maybe instead of rep, leadership, etc determining the size of your party maybe try a different route? Maybe based off your military rank, if you serve, and other factors? A army colonel would be able to lead a bigger unit than a captain or lower. Also if you are a good type of person and play a sheriff or something then maybe honor would play a role. Bandits would need a notoriety factor to get more men to obey them.
Instead of properties and the like have a house or two in every big city or town. You could only own one at a time or at least live at one at a time and they would all differ. Owning a ranch would give you land and other things so you could upgrade em but owning a house would just let you store stuff and rest to get healed without needing to own the town or spending money. Also, it should not cost to rest with a party. They are getting paid regardless so it is up to them to make do with their own money with not fighting, raiding, etc.
If you are going solo, should be able to hire a coach to take you to other places, course there is always a chance that it will be attacked.
M&B is mostly a fighting game but no reason why you have to fight all the time. Sure, it's most of the fun but other venues could be added as well. Maybe buy a saloon and fix er up? A lot of room open for debate on this one.
Target practice. Not sure if this is already a part of M&B or mods, but shooting at target should increase your skill, if only slightly and maybe with luck and some random chance.
Should be able to misbehave in town and then have the sheriff come down and lock up up for a set amount of time or fined or both. Of course you could escape but then a bounty would be made on you. Of course some things would be more drastic than others, for instance, if you killed a man in town.
Various places to recruit from. Forts would offer higher tier troops or let you at least train the ones you gotten. Towns would offer townsmen or cowboys. Ranches would offer ranch hands, etc. All sorts of things could be done. If you belong to an army then you could recruit regular joes in towns but that would be at cost to you (not the ones you get from being a commander or as rewards) and thus you would have to train and equip them yourself. You also should not be able to both sign papers and then join fully a army. Either you sign up for a term or don't. Bandits and law factions would be different though. You could be hired out or join them.
All in all I think I've suggested more than enough right now. Of course some things have already been suggested, others are already being added to the mod, and others still...
Just my thoughts on the matter anyway. More to come, or not.