Whoa, biiig topic!
For starters, the RoleMaster rules, from a "realism" point of view, make me want to pull my hair out. RM isn't realistic. Not even remotely!
(That said, it's a fairly useable combat simulator for RPGin. You have to concede to playing a game at some point, and fighting is not a very important aspect of our campaigns these days anyways. We don't award XP for killing, which goes a long way to make our group avoid senseless and potentially dangerous bloodshed.)
The following is my personal view, based on my personal experience with swordfighting from some 7 years of showfighting (have admittedly given up on that hobby since, though). Been playing RM for some 20 years now.
A few concept ideas for a more "realistic" combat system, compared to RM (somewhat unrefined):
1) You don't fight with a weapon. You always fight with your entire body. A weapon is just an extension of your body. As such, there is essentially no difference to fighting with your bare hands, or with a weapon in one hand, or with a weapon in both hands, or with a weapon in each hand, or with a weapon and a shield in either hand. Also, what the weapon does is improve your bodily functions by adding momentum/power to an impact, improving range, and improving penetration power (through spikes, points and edges).
1a) A sub-point of this very concept is that Two-weapon combo in RM is just... wrong. Using 2 weapons does (should) not give you two separate attacks. You don't split into two, you don't attack along two vectors, actually you cannot even simultaneously coordinate independent attacks with your two arms - thinking it through, why can't you make a full Martial Arts attack with your free arm when wielding only one 1h weapon under current rules? You have two weapon-augmented limbs which gives you additional options and tactical flexibility. To the untrained, this includes additional chances to hurt yourself.
1b) There is no fundamental difference between objects used as a weapon. A rock, a club, a chair, a blade or a shield behave essentially similar which is why the distinction between shields and other weapons plainly doesn't work (except in RM rules).
Sidenote: Whoever assigned a plain +15 DB to main gauches probably never saw one.
1c) Ultimately, the difference between individual weapons is not nearly as big as RM rules suggest. The skill distinction between individual weapons is arguable at best (I really want to say totally stupid). Around two thirds of your combat skill has nothing at all to do with the weapon in your hand, it's all about your footwork, your position and movement relative to your opponent, and generally your ability to use your body (which, as I wrote above, is then augmented by the weapon).
In reality, there are only very few basic attack patterns (punch, swing, stab) and a given weapon will enable you to perform one or more of these better. In our group we have a house rule that says you develop skills in weapon groups, not individual weapons. A club is a club is a club, no matter if the object in your hand is a rock, a hammer, or a mace. They are just different in how much they augment your attack, but the basic principle is virtually the same.
2) There is no clear distinction between OB and DB. DB is always also offensive in nature because you can ignore an enemy attack against an armored part of your body which in turn allows you to attack him instead instead of parrying. By the same token, OB is DB because an attack potential will force the enemy to parry where he could otherwise attack, blocking out certain attack vectors or threatening to trade a hit for a hit.
3) "Initiative" is determined by range first and foremost. Big guys and/or big (long) weapons make all the difference in the world. The nimble thief or evasive ninja is an unrealistic myth. Life just isn't as balanced as some want the rules to be. (Also goes for DB and armor penalties, see below.)
4) It should not be possible to develop hitpoints as a skill. Pain resistance and damage threshold should be a fixed value, directly determined from a stat (CO). You can gain exhaustion points through training, and you can train a certain pain-resistance in real life but that is already covered in "stun removal" or "stunned maneuvering". You cannot train to have more blood or effectively take less damage from a given wound.
5) Bigger weapons don't neccessarily do more damage. Provided that they are not too heavy, big or unwieldy for the user (a real problem in some instances!) they will give different augmentations to your skills and attack maneuvers. At some point, depending on the user's skill and stats, the disadvantages will outweight the advantages.
6) A flat out material bonus does not make sense, much less the idea that certain materials are always flat out better than others. How could a mithril or eog club be more effective than a rock? Mithril is even supposed to be lighter so it should actually incur a negative bonus on bludgeoning weapons. An eog bow is not going to be +30, it is going to be painly unuseable as the string won't bend...
Mind that I am talking about materials here, not generic enchantments (which in turn might require special materials but would function independently from these).
7) The Arms Law hit tables are flawed in their basic premise. The question wether or not a hit was scored should be treated totally separate from the effect of the hit. RM convolutes this into a single table with the added problem of claiming that lower armor values have increased mobility, hence are harder to hit (which actually belongs to DB, not armor). This results in a severe penalty for leather "armor" which actually significantly increases (!) your chances to suffer a critical hit.
8 ) Armor. Oh boy, where to begin.
8a) Again, somebody at some early point of RM got the notion that certain materials are "better" than others, flat out. That's nonsense (I seem to need this word quite often here...). Different materials and armour styles affect different attack types differently. That's why there is such a broad diversity, they all have a balanced set of advantages and disadvantages which combine protection, movement inhibition, price, weight, and a lot of other factors. Ties in with point 7 though: Armor should be able to alleviate or totally negate hits, but it should not affect the probability of being hit in any way. This is one of the biggest issues I have with RM and "realism".
8b) Armor does not hinder movement anywhere near the excessive penalties given in RM. I have seen people in what RM describes as P19 jog for more than 20 minutes to warm up (admittedly, those were our fittest master fighters). In fact, leg movement is hardly hampered at all, but you may find it difficult to raise your arms above shoulder height in heavy plate (the joints block at some point, depending on your exact armor). Higher-grade armor will have more finely worked joints that allow for more movement.
The real problem with full metal armor is the sheer mass of the suit - you will be exhaustet fairly soon. Super-heavy plate (like a Cuirass) is too heavy to bear at all, and requires to move around on a horse.
