Okay, it's still unpolished, but the basic framework is running for those who want to get an early start.
Things you can do currently:
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Create worlds
Add rooms, layers, tilesets, sprites, and gates (entrances only)
Import graphics for tilesets and sprites (no default graphics yet aside from a white square)
Set tile properties. Only basic, solid, sticky, and slick tiles work currently. Open space tiles restrict motion to a grid due to incomplete implementation; they will later block motion normally but you will be able to jump over 1-tile pits. (Note that blank tiles are automatically considered open space unless you set them otherwise.)
Create functions for sprite classes
Display text boxes both through the basic sprite editor and in functions. Text inside {curly braces} will be interpreted as code, so you can put variable references and math expressions in them.
Test your rooms
Set gates as public, and visit any world that has a public gate
There is also program logic for sprite functions, including if, else, loops, and creation of variables. You can target objects with functions using periods, for example a sprite can set globally accessible variables by targeting the room, which is stored in _room. (Example: one sprite sets _room.a to 5, then another sprite can use _room.a to mean 5, since they both have the same room. There are a lot of other things you can do with this but I'll need to make a proper guide.)
Note: All built-in variables will start with an _underscore, so don't create variables that start with an underscore or they might be overwritten later.
Some new concepts that I'm considering on top of those from the old BoundWorlds:
Gate properties and tags - You can set exits to either go to specific gates as before, or to a random one. If you choose random, the game will try to find a public gate that matches requested tags. For example, you can put an ice cave entrance in your world and give it a request to search for public #ice_cave gates. Players entering the cave will then be transported to a random gate with the #ice_cave tag.
Multi-world quests - You can create quest items and set an exit to link to a world that will contain that item; or add "blank" items in your world that will be "filled in" by existing quests. Example: You create a quest in your world to find the Ocarina of Plot, which is in the Dungeon of Spiders. Another player creates an area with #dungeon #spiders tags, and places a "blank" item in the center. When a player goes from your world through the gate that links to the spider dungeon, the Ocarina will be found in it. The game might also have some way of generating entire metroidvania-type adventures using multiple linked worlds. Obviously this is a rather complex process; there will have to be some method of determining whether or not a quest is beatable before generating it, but I'm exploring different possibilities.
A plot (I actually know the plot already, but I need to figure out the best way of expressing it, given the open-ended nature of the game).
World building options becoming unlockable. They won't be hard to unlock; basically this will be a method to teach world builders gradually instead of dumping a ton of confusing options on them at once. Simpler tools will be unlocked first so the player learns the nature of the world building process, eventually leading up to the full scripting system. Right now though, everything is available as I make it.
A procedurally generated "void zone" that links worlds together. Will probably tie into the plot in some way.
There will be a default combat system; I'm debating mainly whether to make turn-based encounters or Zelda-esque action sequences. This will tie into the default physics, which are still not quite set in stone. (Right now it's grid-based with diagonal motion; I'm experimenting with different options that will both feel natural and also be puzzle-friendly.)