Islands are big, and who dosen't like a train or two? It could be like a 2ft steam locomotive (Image for example) with flatcars for players to build on. You could also have boxcars (enclosed flatcar) and gondolas (aggregate transportation. E.G. Iron, Coal, Stone, ect.) Track would be placed like a "spline" (Railroads Online for example) and switches could be snapped to track "nodes" that aren't connected to other track nodes and visa/versa. Trains would be unlocked about when you get steam power. Oh, also, carts and locomotives can be attached and detached.
Now, I bet we are all tired of manually making thousands of nails and whatnot. So, what if we could automate it? You could make it kind-of like factorio, but you could have a module to put on default crafting stations that requires energy, but can craft automatically. You could have inserters that would transfer items from object to another object, and conveyor belts that transport items.
Now, I noticed that free-placed objects won't grid snap to other free-placed objects. Meaning, I would like to see a independent grid for free-placed objects, so we could build custom stuff easier. Or I'm just missing something and it exists.
Ship Automation: Essentially a device that will pilot the ship to/fro diffrent islands, and even regions, and load/unload it's inventory into a "dock"
Subersibles: While messing around in Creative, I saw a wreaked sub. What if, we got submarines (Like the main vehicle from FAR: Changing tides.)