I'm really glad you're adding more game elements!
This sounds like a really good system, although I can imagine players could get annoyed with everything being permanently lost and feeling like they need to keep playing just so their world doesn't get deleted.
While it could be an issue if deletion happens to frequently or fast, a world will only be deleted if (A) its creator stops playing/earning coins AND (B) people stop visiting the world. Coins earned by a world go directly toward that world's upkeep - if a player makes a decent creation and someone else links to it and players start using it as a passageway, it will remain as long as players keep using it. Even without an official 'safe' option, once this happens enough the chances of a world being deleted are virtually nil.
Possibly the worlds could get 'corrupted' instead of deleted, you could delete some random blocks/change some and just make it a mess. This could be undone with a certain number of coins perhaps.
Don't like it. The complexity of the world building system means that changing things randomly would do more than just make the world a mess, it could lead to the world becoming completely non-functional or even cause crashes. Besides, the point of the deletion system (besides being something to do with coins) is to free up database memory that is being used by junk worlds - messing them up even more without deleting them would be counterproductive.
A player 'home base' map, which they could decorate as they wish and would start at each time they logged in. This couldn't be visited by other players, but could serve as a sort of playground for them to design at will and give them somewhere to just relax.
Not a bad idea. You can already make private worlds by just not giving them a public entrance, and travel to them/test them through 'My Gates', but allowing the player to 'escape' directly to a designated location (one that, perhaps, has links to their own favorite worlds) is certainly an option.
Another type of more valuable resource, perhaps one that's only gifted in a few rare occasions or could be bought for a highish number of coins. This could be used to do special things and would be something that people would really strive to get. Perhaps it could allow them to keep worlds completely safe, or perhaps it could be just used to buy certain types of stuff for the home base.
Hmm... well, players will be able to make their own permanent items, that can be used for whatever they want within the confines of that world. Some sort of 'shop' system could work though. Or perhaps, just as coins a player gains are reduced from the total amount they would give to the world on exiting, coins a player spends could add extra to that world's upkeep. There would have to be some means of ensuring that world builders wouldn't simply place hidden tiles that steal coins from whoever walked over them, though...
It's hard to think of special in-game items that could be 'purchased', since everything that appears in the game, players will be able to make themselves! The only real resource is the time and effort it takes to build them.
Some sort of threat. Currently there isn't anything that can really damage the player, and it'd be good if there was some sort of threat. It'd be good to see some interesting mechanics for this, instead of just a few random monsters put down.
You can actually make enemies, although you need to use the 'advanced' mode. In the enemy sprite's 'collision' function (which runs whenever the sprite steps onto another sprite's tile), make a conditional block that checks if the other sprite in the collision is the player (if @ == *), and if so, make the player jump to wherever you want on the board - back to start, back to the beginning of the room, or make them jump with 'collides' active and have them land on a sprite that will transport them to another room. You can even set a global variable to use as hit points and run a check that will do something different if they all run out. It's very open-ended, although I can imagine that enemies are something players might want to have in the basic function editor.
The chaos causes monstrous shadows to appear, these appear as you gain more coins (to stop beginners having to deal with them). These could start off randomly pacing around like sort of random ghosts, but could then get more purposeful in following you/attacking you as you gained more and more coins. Walking into them would cause damage (or a loss of coins or however you want to deal with it). These could then be negated/set back with help of the valuable resource. This would also happen on any world other than 'safe worlds' which could be designated with the valuable resource or some other way.
Don't like it - see above for the reasons against having the program mess around with other people's worlds. The 'plot' is more of a framing element to describe why the coin/deletion system operates the way it does, but it should remain strictly meta. If world builders want to use it as a part of their world's story, that is up to them.
Lastly, I would perhaps suggest changing coins to something else - coins are very mario, and I don't really see them fitting in with the chaos theme.
Yeeeah... I was kind of excusing myself because coins are also a generic symbol (in Tarot, anyway) for the stability of the physical world, buuut... mostly because I just couldn't think of anything better. Of course, world builders can make them look like whatever they want.