My plan was to use the ontons invested in a world to determine its overall 'popularity level'. In general, the ontons invested in a world will be a relatively decent measure of that world's overall popularity and enjoyment factor, since worlds naturally accumulate ontons by players spending more time in them than the amount of ontons they take away, and making it to an exit (as opposed to quitting). It also enables some player-driven flexibility, since a player can 'promote' their world by investing their own ontons (acquired through either exploring or building another popular world) in it.
Of course, this does give a benefit to long-time players, so there would have to be other factors involved. There will certianly be a newly created/recently edited list in the Universal Gate, as well as other options like 'newly rising' and so on. I'd like to give some sort of benefit to players who help promote new worlds by creating links to them, but I'm not really sure how to balance this.
At some point, there will be a system for purchasing or acquiring items (which, at its most basic level, is just a variable that is retained when you leave a world), and at that point any item could potentially be used as a kind of 'currency'. As for players accessing items created by other players (for exchange rates and so on), I would like this to be possible, but there are a lot of conceptual issues to deal with before this will be viable. What if they create have a tile that you step on and it invisibly takes away all of your money (or the Crystal of Plot Advancement) from another player's world?
This can be avoided by explicitly setting items as private or public, or creating a hardcoded shop interface that ensures players can't have their foreign items taken away without their permission. But even this leaves open the possibility of someone setting an absurd exchange rate, charging a hard-to-acquire item or a bunch of money from another person's world for a worthless item disguised as something important, and other feats of trolling that could reduce the game's fun factor overall.
As for using ontons as currency, this should be a possibility, within limits. (Actually, I think that ontons should be the smallest possible currency unit.) However, this must be done carefully. Ontons are not technically 'created' or 'destroyed' by modifying their variable, they are created by players spending time in a world (currently one per second, this value may be modified in the future) and destroyed by the World Devourer (meta-plot stuff, but basically ontons invested in a world will be gradually reduced over time, mainly to get rid of low-quality content). 'Acquiring' ontons while exploring a world puts a certain amount into your account, and all of them you created and do not 'take away' are added to the world's account instead. This should be the only way of 'creating' ontons; there should never be more in circulation than the total amount of time all players have been playing BoundWorlds.
Normally this isn't a big problem, since it basically falls on the world builder's shoulders to limit the amount of ontons they will 'give' a player; just don't make it possible for a player to get more ontons than the amount of seconds it took to obtain them. But if a player can sell items in exchange for ontons, they might take a short trip to a world, sell a whole bunch of vendor trash at once only to have all of that hard-earned money be swept away into the void the moment they step back through a Gate. Buying items for ontons is more reasonable (provided it was done through a hard-coded interface to prevent automatic onton-stealing tiles), but still makes it possible to cheat players by setting up shops full of worthless junk.
In the end, it might be simpler for world builders to just use their own world's local currency and set up an exchange booth near their world's Gate to handle conversion between local currency and ontons. The time the player spent in their world will be a readable variable, so it will be possible to make those kinds of calculations if you know how the system works.
(For those wondering, it is not canonically important what you call ontons; that's just a name we decided on in the last topic to make discussion about them simpler. Individual worlds might call them coins, gems, energy units, whatever. As far as the game's code is concerned, they are just '$', and as far as the meta-plot is concerned, they are...well I don't want to spoil it too much, but suffice it to say that their origin is too incomprehensible to be given a proper name.)
All or most variables used by the system will be readable in the new version, so it will be possible to read the name of the world a player last visited and display it on a sign, because the 'returning gate' system retains information about the last world a player was in.