Id like to see the targeting and lack of friendly fire removed from melee, I suppose like Darkfall.
Id like to see the Inventory system changed from a sot based system to a free-form system like Ultima Online
Id love to see mounted combat and ranged combat work very much like mount and blade. Mount and Blade's combat system is both incredibly fluid and realistic but it is intuitive and easy to pick up.
Id like to see torches implemented well, like in oblivion
Id like to see random events take place when you gather resources similar to Runescape, (ie, the player has been chopping wood for a long time, eventually an Ent or something will spawn that the player has to kill. This would make menial tasks interesting and make macroing difficult.
Id like to see house building similar to the way Ultima Online did it (maybe a little less complicated), where they player can lay out the floor of the house then add walls and a roof of what ever material they want. And then place doors, windows, balconies etc into the house, before finishing it with furniture.