I once wrote these feature ideas down for a non fantasy medieval mmorpg (for some reason; way before M:BG was announced):
- fully realistic fictional medieval world (no fantasy items, beasts, just land names different)
- Mount & Blade -ish combat system
- skill based leveling (skill increased by using appropriate actions)
- combat skills like one handed swords, two handed swords, shield
- other skills like crafting, farming, fishing, tailoring, resource gathering and many more
- skill caps and limitations to prevent grind to win
- Player control of villages, castles, lands... (setting tax, amount of guards, …)
- Players earning titles by raising skills (good farming skill = “Farmer” Joe)
- Royal titles such as Count, Duke, King (ala CK2)
- Royal titles earned by earning prestige (battles, efficiency of owned land, buy with gold)
- Player title hierarchical structure, Counts paying taxes to Dukes, etc (just like CK2)
- Player run economy
- Every item can/must be crafted
- Localized trade, no global shops/auction houses, transporting goods by yourself (with cart, ..)
- Resources depending on counties and land, each land would only have certain resources which will encourage trading
- Open PK PvP, killing without reason = murder = consequences = guards alert at villages
- Full loot on death
- Outlaw system, outlaw camps/villages, jail system for those caught (pay or mine in a mine, hand in ore to get out)
- Faction warfare over kingdoms/lands, Kings declaring war costs prestige etc., killing in wars != murder, wars/sieges scheduled
- Land claiming, house building, sandbox features …
- Tournaments for earning prestige
- Buildable / customized castles / villages (you'd have to build and place blacksmith, walls, towers, etc)
- Kings, Duke, … lose throne by losing prestige every day of control, then they can be challenged (in a tournament, by guild war, ..) for the title
- Item quality
- You can sell to NPC villagers, but it is discouraged (less money and it takes time) - selling 100 wood for example in your shop would take 10 hours, 10 wood per hour. Selling to players would give you more gold and faster time.
- Possible permadeath and character retirement, you then play as a son with about the same skill/attribute stats
- Faction switching with severe penalties