The new physics system is up.
It might still have a few bugs but overall should be more consistent than the earlier system. The main point of the new system is to be more spatially cohesive; anywhere you can cross from one tile to another, you should be able to cross back by going the same way (barring logical barriers like gravity). Anywhere that this rule doesn't hold up is a bug and will be fixed.
It's more restrictive than before. Boundworlds was not intended to use a seamless blend of top-down and sidescrolling physics; each area should be either one or the other. The new physics should encourage this - there are certain geometries where it's possible to cross from top-down to sidescrolling, in other locations drop tiles will act as barriers. You can drop in from above, climb or fly up from below, and cross from the side on tiles where there is a clearly-placed solid floor in the sidescrolling region at the point of entry.
Ladders no longer work automatically when you cross their tile, you must press the up key to grab a ladder.
The only existing rooms where the new physics pose a problem (that I know about) are Trials 2 and 3 in the Treasure Dungeon, but they can be pretty easily adapted by replacing the floor tiles with solid tiles.
The items basically would all send you to a central world (the hub world of this concept, basically), from which gates could be taken to other relevant worlds. The items that would get you to the central hub would be obtained in any world with Elen and Alend (I should consider new names, but I am atrocious at names. Of the names used so far, two are based on names from a book (although the characters aren't), and one is a terrible pun based on a french word (Batelier from atelier). But, back to the topic), or from one of the dungeons I make (exclusively if it is relevant to the project. For example, my current dungeon project is planned to give you an item for completing it that will open a room in the hub world if complete twoish other worlds), or from a couple other things in worlds I make (Putting gates at the end of each of them wouldn't have the right aesthetics). Don't know how well this project will turn out, but if it goes well, it should be neat. I am hoping to spread the lore throughout and to have neat dungeons. It probably won't turn out well, but that isn't going to stop me from trying.
All of this will be doable once item transportation is implemented - the only questionable bit is the items transporting you to the hub from other players' worlds without the world builder permitting it, but they would work in any of your own worlds. You can already do this, kind of - just make the "use" function send the player to the gate of your choice.
To "share" an item between worlds, even your own, you need to package it; this will allow the same item to be accessed by multiple worlds.
If the dungeons are "open" to public gates, players might run into them at random; they could return to the hub at the end of the dungeon (if they have the transport item) and use the items they collect from each dungeon to unlock new areas in the hub.
One possibility is to make a teleport pad that does nothing unless you have the teleport item, but sends you to the hub if you have it. The hub could be a private area, but there would be a pad at the end of each dungeon, and maybe one public area with an easily-accessible pad.