please note that it would be boring to just cut and paste LOTR, so some ideas i'm making up off the top of my head.
you were a human brought by ship toward mordor, your a slave being sold by other men to sauron. the ship is attacked and liberated by elves who take you ashore but are quickly ambushed by orcs wanting revenge.
you are forced to run, and there you are, in middle earth open world goodness you see the misty mountains in the distance, the dark red glow to the south.
/end random story plot thoughts
now if bethesda made it we know it would have huge open world, and epic quest to go on, but it doesnt stop there. If you think about it there would be many cities and dozens of small towns or villages ripe for saving, or pillaging.
you could decide to join the forces of good, or join sauron. or you could go find the hobbits and take the ring for yourself leading to much re-playability
cities and towns could be attacked at random, you see smoke in the far distance and you know there is a battle, dungeons would be crawling with goblins across the floor and on the ceiling
there is so much bethesda could do with LOTR its scary.
but, they don't have the rights to it.... /sadness.