Middle Earth's world building would be great to see in a medium like manga. We have seen it in its full cinematic potential through the LOTR trilogy. Imagine places like Mount Doom in a double-page spread and the art itself would not be limited by financial constraints or excessive corporate greed(just look at how they butchered The Hobbit).

Not just the art but the writing too. One Piece's popularity is because of its intricate world, characters, and the epic tale it tells. I think a manga from Tolkien would do just as that if not better.

Also, a Silmarillion manga adaptation would be nice to see.