I do things like this all the time with computer games. I find that oftentimes the devs have included various features that make a game very easy, so I have to create my own limitations to add challenge to a game. These aren't house rules however. I prefer to call them SIRs (Self-imposed Restrictions). Here are some of my ideas for M&B:
1) Unconsciousness = death for non-nobles
In the game, nobles never die. Why? I imagine it's something like D&D's death's door rule. You don't immediately die when dropped below 0 hit points, but you continue to bleed to death unless somebody helps you. I imagine that nobody bothers to help all the commoners littering a field of battle, having more incentive not to ("I loot the bodies!"). Lords however can be ransomed by enemies, or give fabulous cash prizes to those who've saved their lives. As such, there is an incentive for both friends and foes alike to make sure that lords survive battles.
The point of all this is that when you, a commoner, are knocked unconscious in the beginning of the game, you should consider yourself dead. Nobody's going to save you, nobody cares. What troops you have with you will loot your body, grab your purse, and move on to the next tavern, where they will quickly spend your gold getting drunk. "Bu... but, I'm the PC!" So what?! So what if you pay them a couple of coins a week, you think that means they love you? Nobody gives a **** about you, so they will leave your body to rot. If you were an ass, they might even rape you first.
Every time you are knocked unconscious, for any reason, you must start a new game. No exceptions, no excuses. No, "I didn't mean to click there," no, "I didn't want to fight them." This continues until such time as you are a lord serving a king. Only then will you survive being KO'd, because then people will have reason to make sure you stay alive. This makes the spy report mission for example, quite dangerous, since if the guards notice you, you only have robes and a quarterstaff, and unconsciousness = game over.
I spent two weeks real-time starting over again and again because my guys didn't survive. Not everyone might find that fun, but it lets you say that when you do make it to lord, that you really did LIVE that long.
2) You must be a captain before a lord
I think it's silly that just because you took out a few packs of looters and bandits, suddenly a king will personally invite you to take charge over his lands and properties. What does dealing with vagabonds have to do with running a fief, managing peasants, or promoting trade? Who's to say you didn't even just pay some troubadours a few coins to say you killed bandits? Ennobling a commoner is a very rare thing and not something a king will do lightly, certainly not over rumors and hearsay. He needs to see firsthand what kind of a person you are, how well you take orders, and how useful you are in REAL fights, with other lords.
Therefore, before you can become a vassal, you must serve that lord as a mercenary captain for at least three months before you can accept vassalage. Only after the three months is up, can you then accept an offer from that king, and that king alone. You cannot ask to be ennobled, you must wait for the offer to be made. And you can only accept offers from kings that you have served as captain first. Think of it as a trial period, where the king and other lords can check you out firsthand, and see what you're all about.
3) Ranks before recruits
When you start out, you're a nobody. People don't follow nobodies, and there may be laws against commoners raising large military forces. So at the beginning, the only help you can hire are companions. You cannot have troops from anywhere else, only companions can help you.
Once you become a mercenary captain, you can then go to taverns and dun dun dunh... hire mercenaries. The contract and oath you give to the lord and to the king are what gives you the legal authority to start building your forces. You can only hire mercs, and mercs are expensive, but it's a start.
Once your term is over and you have been ennobled and given a village of your own (knighted), you can now hire recruits, in addition to still being able to hire mercs. However, you can only get recruits from your own village. Poaching peasants from the properties of other lords was considered very bad form, and could lead to wars, so it's a no-no for you. You are still limited to using only mercs and units from your own faction.
When you finally gain a castle (baron), you can start using troops from other factions. You can't recruit them, you can never do that, but you can start using prisoners you've rescued from defeated enemies. Other lords will no longer question your loyalty when you start using cross-faction units.
At the last stage, gaining a town (dukedom), you can now hire recruits from any settlement of your faction, not just your own properties. Your wealth, power, and political influence are such that other lords aren't going to hassle you over taking a few serfs to serve your your armies, especially if their lands were acquired through your efforts in the first place.
4) Stats and Skills
Every stat has certain skills associated with them. When raising a stat, you must raise one of it's corresponding skills, and vice versa. If you want to boost leadership, you must raise charisma. If you want to raise strength, you must boost one of it's four skills. This should result in a character that may be more "realistic". In any case, it should make you think a little more about how you develop your PC.
5) Ruined gear = lost
The only accessories you can really lose in combat are shields and horses. So, if a horse you're riding is cut down from under you, or your shield is broken, then you must drop those things from your inventory after combat. Ammo runs out, but there's always so much ammo lying around after combat that I don't see replenishing that as a problem. Besides, shields and horses aren't that expensive, but replacing them will be a drain on your funds.
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I do like some of the ideas of the OP which I was using in a similar way, like #'s 4 & 5.
There's also another idea you could try. It's unrealistic, which is why I didn't list it above, but nonetheless challenging, and one I've used many times in similar games: a no-shop rule, or scavenger rule. This means that you can't buy any gear from stores. Everything you wear / wield must be scavenged from the battlefield. Try getting your reinforced plate now! You can still buy other things, like food and textiles, just not arms / armor. This may make Looting a more valuable skill in your game.