SORRY FOR THE BAD ENGLISH, I'M ITALIAN
I had an idea. I usually see around MMOs that the plot is to reach the top level and kill everything.
My idea is this:
-The player is you avatar, so he's fully human (so he can get hungry, have feelings related to the ambience or other).
-Levels are set by experience, let me explain... You have to learn to do an action, mining for example, you have to go to an NPC that learn it to you, but you have to practice to become a good miner.
-It's a sanbox game (Open World), so you can travel through the whole world meeting deserts, forests, glaciers, mountains, plains etc.
Probably during the trip you can meet other people playing, NPC (that can give you quests, like most of the games around). You can meet wild animals too.
The world needs to be large, but not immediatly, when your avatar "born", you have to choice a city where to born (or generated casually), so in that city choosen will be a new house where your avatar will live.
-The game can be setted in a world where the level of civilization it's divided into metropolis, small towns and villages.
-The technology level it's like ours.
Now I have to go, i didn't finished yet (while i'm writing i'm additing something new to the idea). I'll post soon.
Let me know what you thing about it. Bye!