Something I just noticed, the OP hasn't been updated with what we have thus far. Perhaps would be a good idea to update that. Second thing, I've thought of drawing up a rough visual of the dimension's look.
For the future: Third, and I cannot stress this enough; avoid the fallacy of artificial difficulty, and forcing gimmicks. I see this all too often in suggestions even far outside of Minecraft. Example, if someone works their butt off to get an Elytra, they better darned well be able to use it, considering both its rarity, difficulty to obtain, and position in the end-game. When we finalize the structures themselves, their rarity needs to be reasonable as well. Going thousands of blocks for a single structure is far outside the realms of decent design. Remember, folks have limited time, and others may not want to spend hours just roaming for a structure. This would be more agonizing when locked to only a single mode of transport. You must also keep in mine Minecraft is a very multiplayer game.
Future note for what to brainstorm for this dimension, due to its nature as being separate from a linear progression:
-What is annoying to do in vanilla?
-What would be a cool, non-damaging thing to have around? A tool? A sort of utility mob?
-What (non-mineral) materials are hard to get normally and often get used a lot in building?
-There can also be ways to upgrade certain vehicles, tools, and such, when combined with materials from other dimensions.