Honestly, I think that most dimensions aren't well thought out and are either too simple or too complicated.. Aether has the right idea with all the concepts and new things, so let's talk about that. The reason why Aether has such a success is it has all new items, blocks, mobs, even the inventory has been changed. It also takes notice of the nether portal and makes a reference to it with its glowstone and water portal. I think the thing we're going for here is something that's great and different but doesn't stray too far from what the game truly is. My suggestion would be a world of a dungeon where you can collect all kinds of new items, going through portals to get to the next levels after fighting bosses. But make sure the sprites and coding aren't too far from the original minecraft, or it'll turn out bad.
~~Ethan
