Players need to have Goals when playing, otherwise most of them will get bored by the Sandbox Style.
So we are gonna need some Stuff that people can actually do even in Singleplayer.
Creatively build Stuff that looks good and share it with others. Not really much to explain apart fro mthe Building Blocks you can use, but those are mentioned already.
With the Random Procedural Nature of the World and its Blocks, Plants, Ambient Music (that one would likely not be procedural), Mobs, NPCs and Items, you will rarely ever run out of brand new things to find.
In the first World Type I want to add, there will be countless Island Regions, each with its own set of random things to find out about. Kindof comparable to the Grand Line in One Piece.
Maybe you dont want to leave your Region, or you want to turn this whole thing into a spiffy Tycoon Game, so how would you get all the nice things that Explorers typically get?
Simple, just have NPCs from other Regions visit your Town and sell you the things unique to their places.
The larger and more known your Town gets, the further the NPCs will travel to get to it and sell Stuff.
This will be mostly defined by how much Resources you export to other places.
So you can still have a big and potentially secret City, but next to no trade going on since nobody knows what you could provide to them.
All Towns in the World will start off as non-trading Towns, but Towns in unvisited Areas will increase their Trading Capacity if you export to them.
Do note that certain Towns are so hard to visit that you wont be able to trade with them unless you actually explore yourself find them.
You want to advance in Tech to become the very best? No Problem!
Not only can you achieve all the essentials within a single Region, since all Resources exist everywhere in some way shape or form.
But you can always explore or trade for better Resources, specialized for optimizing your Technology even more! Imagine all the Combinations of things you could pull off!
Mixing and matching Alloys made of Metals from different Regions at different Ratios, until you find one of which you think it is good, and then coating or galvanizing that alloy to prevent Rust, or kill Werewolves or something.
Combining Ingredients from all over the World for making Food, Medicine or Potions of high Quality.
Living the City Life
You just want to live in a Town and be a normal Citizen? No Problem, you can do many things here too.
Working, Guarding, Managing, Ruling, Farming, Trading, Delivering, Performing (in some Music-ish way), Researching, Stealing, Murdering, Assassinating, Criminal Activities and more will be doable, just like in a real City.
Ofcourse just like the Trade Route, the Town will grow into a City, so that more things are done by NPCs and the Place prospers.
While it might be hard to implement Armies in a Minecraft alike Game, it is definitely doable for someone to become Strong enough to just take over a Town using their own Wits and Power.
Once you achieved ownership of a Town, you can tell people what to do, and later on might be possible to make yourself an Army and attack another Town to take it over too.
And like with trading, you can just have the Resources from Places you took over, since you own them.
I am not sure if you are going to have to generate/visit the Towns yourself to start an actual Invasion. I do not want some janky Auto Resolver Bullshit, that is either never accurate or way too predictable.
Strategy is a very important thing for this, you could sneak into the enemy Town and just take it over from the inside if they dont notice you first. (digging into Town will very likely be noticed by the Townsfolk!)
Decided to write down some Goals people could be going for in Mechaenetia. ^^