I would probably combine Mount & Blade: Warband (modded with Diplomacy) with games like The Guild 2 and, in some small part, The Elder Scrolls 3 and 4 (but without magic) and implementing the ability to make your own things from the simple wooden cup to an iron sword to your own house (eventually, as these things take time) from the raw materials available to you.
It's possible that a game like this is already in the works, but still. To have all of the options starting from the medieval era on with all of the technical and economic status available to you. You'd have these options at the beginning of the game:
1) Starting off as a thief. Possibly the hardest to start off as because you'll need to pick pockets, break into houses, or just outright beg so as to get your daily bread, but pickpockets were as common back then as modern thieves are today, and there are plenty of gamers out there who prefer to play as this type of class [yours truly included =P].)
2) Starting off as a farmer, be it a serf to a lord or a free person living on a lord's lands, sewing and planting his fields while raising a family plus the low chance of serving in his army (if you're a serf) and, thus, gain some military skills while in his service (you'd get paid a small amount each month on top of the high risk of getting killed on the battlefield.) As hard as playing as a thief, but at least you can feed yourself.
3) Playing as a merchant, going between towns and villages and trading off your goods. This class is a mediocre start, as you get a decent amount of money, but know very little about defending yourself and your caravan.
4) Starting off as a hardened mercenary leader and thus become quite valuable as part of a war party or castle defence.
Remember, the life of a merc was very hard back then, as there was always someone new looking for adventure and were more likely to be hired. Many would never make it out of a battle and, as such, the person who hired them would never have to pay a single coin on them. This is also a mediocre start, except you don't start off with much money and you have to rely on being hired by others (traders looking for caravan guards mostly, though you'd have a bigger payout through fighting for someone else in times of wars. Either way, you'd have to survive if you wish to get paid.)
5) Starting off as a named nobleman, with all of the privileges that come with. If you choose this class, then you'll be further asked which country you want to start in and which title to you wish to hold except for king/queen of that land (example: if you start off as a noble in England, you can choose to be a Count or a Duke, and then are prompted to choose which lands you wish to hold for the king.)
Like Warband, if you choose to get involved in any fight, then you'll be fighting alongside other people. You can choose to swear Vassalage to a king and be granted land to hold for him. However, you're given the lowest title of Lordship (example, Count of Essex), and if you wish to be granted a higher title, then you'd have to perform favors for your lord and/or king, and usually that's given by fighting in battles with him.
You can also eventually build up enough money that you can purchase land and build your own farm, workshop (this can be a tailor's workshop, a carpentry shop, a smithy, or a jeweler's workshop), hospital, or Pub/Tavern/Inn and make your living that way (there would have to be of a certain level of proficiency in the skill needed to run those kinds of businesses, like smithing, crafting, all three of the medical skills from Mount & Blade, carpentry, brewing & baking, and sewing). If you buy land for a farm and then start up a business, then you would be able to provide the necessary materials for most of the listed businesses. However you'd have either buy wood from the nearest market for many of the carpentry projects or send your workers to chop the needed wood, and as far as metals and precious gems go, you'd most likely have to buy them off the market. However, as the game enters the Renaissance era, if you have the coin to spare you can purchase specific mines (iron, silver, gold, ruby, etc.)
You can also raise a family by wooing a woman/being wooed as a woman (I'd rather have the option of being a woman, though this'll make playing the game that much harder [heavy amounts of sexual discrimination were present around that time], though it's not unheard of for women to have jobs that didn't involve housekeeping and whoring) and when you pass on, you take control of your eldest child. However, if you die childless, then control goes to your closest surviving relative (towards the beginning of the game, if you've married and somehow died early, then that closest relative would be your spouse), and the game'll continue from there. However, if you've never married, then it's game over for that character (unless you keep save games from early in the game) and you have to start anew. You have to make sure that your family is fed, and if you, as the head of your house (if playing as a male) or your spouse (if playing as a female) then your family'll starve and probably die (the chances of any given member surviving starvation decreases everyday that he/she are not fed.)
Needs to have a multiplayer option without limiting all of this game's aspects!!!
And one last thing: Playing the game in 1st person and 3rd person views and traveling either on foot, on horseback, or driving a caravan. No overview map to use to find towns and a close-up mini map to help you find specific buildings withing a small area (I'd say 50 feet in either direction sounds reasonable), and the only way to find towns is to follow road signs (unrealistic in the medieval times, but this is so that you don't get lost in the woods or anything.) Also, the ability to attack while on horseback.
This, in my opinion, is my dream game.
It's possible that I'll edit this post later considering how long this post is, as I might have left some things out.