I was thinking that you could make a world with several generated structures not in vanilla, or maybe with veins of random blocks generated like ores. (melons, pumpkins, End Stone, logs, etc.) Also, I think I do see unusual terrain in the current Outworld.
I have to admit, I never thought to have veins of melons — lol — but this did remind me of some ideas I had about creating some different underground terrain layers. We did that with one or two outworlds, but there’s still more things that could be done that would be interesting. Endstone is a decent idea. Maybe a custom biome could replace stone with endstone.
For the immediate future, we have two outworlds planned, which are going to be vanilla and amplified. After every new update I do a vanilla outworld before going back to custom terrain generation.
If you want to help create some exciting new biomes and terrain for the server, experiment with TerrainControl. I could use some help with that as it takes time.
If you want to make some nice structures, they need to be in 16×16 in size, as Ray said. For connected structures, each segment must also be 16×16. We have some demonstrations over near “/warp create”. I can even give some chunk plots to you if you have the desire to try your hand at it.