Ok, the stuff you ask is indeed way down the line, but you have some interesting questions in there, so i’ll try to answer them.

1) I have been told that the LU client can be modded, and we know some places where we can decode and possibly manipulate the data, but it requires a lot of experience as far as I know. Furthermore, doing a full update like the fire temple is difficult for two reasons: We would need to add completely new Worlds/Scenes/Levels to the game, which is likely referenced in a lot of placess, making it very difficult. Additionally, we would need to do some work to prevent „outdated“ clients from connecting, ideally by changing the version, which is likely not easy either.
TL;DR: Modding details is technically possible, new update is difficult and way down the line.

2) Art and design for such a level would need to be done in the format that the original LU uses, since the client has a lot of gamedata included and needs to be updated for new content too. If you are interested, look at .nif files or the Gamebryo engine (be careful though, i’ve heard they don’t like when people upload LU stuff to their systems). If we do such a thing, designers would be appreciated I guess, but since it’s long in the future there is absolutely no reason to do anything in that direction at this point, since we don’t even completely know what we need for an „update“.
TL;DR: Yes, why not have designers, but at this point, no use for them at all.

3) Private Islands/Propertys shouldn’t be that hard to own a property, what is hard is the thinking cap mode to actually build BBB (BrickByBrick). However spawning objects with commands might be possible if someone takes the time to implement it when we have object loading from level files done, which goes hand in hand with NPCs.
TL;DR: Property owning is easy, building on it is hard. And you might miss some GUIs.

4) The LDD has the data for all those bricks, but the client has them too, in the format we need them in. That’s all we need as far as I can tell, so no need to pull apart LDD. As for the ‚Master Builders‘ thing, the finished models too are in the client files, we can reuse those, adding new ones is not easy and very low priority.