Well, what games did you already play? That would help us give recommendations
Also, while not a RPG in the strictest sense, maybe something like Mount & Blade? It does not have a grand story, looks dated and is not easy to get into, but it has a deep customization and a really big, living changing world. The RPG aspect would be more living as your character inside the world instead of experiencing a specific, epic story, but it would fit the medieval mood as well as all the other criteria you mentioned.
It is not for everyone, but the world experience is incredibly deep because you can handle so many aspects of the world in the game including large scale diplomacy and mass combat and it has a vibrant community with a lot of mods as well.
Best of all, you can check the demo, which is not time- but level-limited from here and see if it fits what you want:
Taleworlds Homepage (Developer of Mount & Blade) [http://www.taleworlds.com/]