I have been a wargamer since 1972 and as a consequence, have learned much over the years regarding how military units work both in campaign and on the battle field. For this reason I have routinely had the same criticisms of various mods I've played over the years. Since you are all just starting, I hope to save myself some time remodding you myself. This is a good thing because; first, I only know how to mod a couple of things; and second, I can't mod everything that needs to be modded.
Movement allowances. Your typical Roman Army at the height of the Roman Road system could, with a full supply train, march all the way up the boot of Italy and down to the city of Corinth in six months, which would be the time frame for the two turns a year.
Without a supply train, that army would be at the eastern border of Anatolia looking down at the Mesopotamian Plains.
Now the road system was not as extensive in the Middle Ages, but that should give you a pretty good idea as to how far your armies should be able to move. If you can not, as the English in 1066, march from London to York and take out a Norwegian Army, and then march back down to Wessex on the southern coast and get an arrow in the eye from the Normans in a single summer turn, your movement allowance is too small.
Finances - The expense of the army was always far greater than the expense of building. While cathedrals and walls would take years to build, the moneies spent were seldom beyond the local financial capability. The army on the other hand, broke the yearly budget of the entire kingdom. In mod after mod after mod, you pay thousands to build your infrastruction and hundreds to raise your army. In real life it was the other way around.
It should take five turns minimum to build a building, and often ten or twenty turns, but the expense should be such that it would not be that unusual for you to be building something in every single town, castle, city, etc. that you have once you get yourself established.
Likewise buildings will never take as long to build as walls, roads, port facilities, or agricultural improvements. Those things require massive amounts of manual labor to achieve and the success of the Romans was due to the fact that these things were undertaken by their armies when not out campaigning.
Armies on the other hand are going to be very expensive to recruit and maintain. While peasents and other 'grab 'em and stick a spear in their hand' troops will be cheap to raise and maintain, your professionals are going to cost you much. Sun Tzu was correct in that it took the entire financial capability of 8 men to keep one man on the field in an army.
Thus your financial situation should be such that if you have a total population of 80,000 in your towns and cities, you will have sufficient financies to raise an army of 10,000 of which 5% are elite, 15% are professional, and the remainder 80% are levies. In practical terms, five stacks which have 100 knights, 300 men at arms and archers, and 1600 peasents or militia spear. And that will cost you everything that you are going to earn that year in income to both raise and keep on the field those five stacks.
However, when not out on the field, the army should be practically free. I would propose that every single unit be free to garrison and have the garrison allowances of sufficient extent that you could easly put those five stacks into ten towns or castles and not cost a thing to maintain. Naturally villages will not be as good to garrison as moat and bailys and only the largest citidels or cities will be able to garrison 20 units each, but they should be more extensive than the vanilla and this would reflect that cost of keeping the army out on the field which numerous scripting efforts have produces such odd financial situations which are not quite real.
Provences - I like lots of provences, who doesn't, but one of the things I've always felt was a bit off was the fact that the further east you got, the larger the provences got as well. While this is done to reflect the population disparity, there's a better way to do it.
Provences in my mind should all be roughly the same size and the eastern one's be almost all tiny villages which will take forever to build up. Thus you will have an entirely different set of issues when dealing with the eastern campaigns than with the western since you won't be able to build up the infrastructure in the east like you can in the west. Naturally population growth will need to reflect this. It needs to be as slow as you can make it.
Campaigns - The persistant fear is that of a siege fest. Folks, like it or not, this was the situation in the world until the 1800's when gunpowder technology reached a point where fortifications were unable to absorb all the damage an army was capable of inflicting
In a realistic mod, your typical army is going to be out there laying siege to a well defended castle or town and hoping that it falls prior to the arrival of the relieving army nearly all the time. The battle on the open field isn't going to be the rule, but rather the exception.
This is why the fast armies had a moble siege train with them. Or the tools and parts needed to build the siege equipment on the site. The AI should be concerned about making sure each and every one if it's sites has a decent garrison in it to make sieges long and costly for it fhe garrison is to low then the assult is worth the casulties since they won't be that low.
Area of Recruitment - This is a must in my opinion. Each culture had it's own ways of fighting and certain units should be available to anyone who happens to control that or part of that territory. The English made good use of the welsh longbow, but it wasn't England which made the welsh longbow, it was the Welsh of Wales who ended up being controled by England. Had the Irish taken over, it would have been the Irish using the welsh longbow.
Likewise mercenaries should have a period of time in which they show up and remain recuitable but also fade away. You won't be able to hire Swiss Halberders in 1225, but in 1525 you want as many as you can get your hands on. So mercenaries should be plentiful but likewise expensive.
Well that's all I can think of for now. I'd love to volunteer for this mod since I am quite intriqued by it. But I can't mod much and my historical expertese is limited to the theology, philiosphy, and the overall history of warfare. I can offer advice on general topics, but on how good the spearmen of the Lombard League were in 975, I don't have the info nor do I know where to find it.