It sounds fun. Would it be a GUI like you were planning like an OpenSim type deal that is rendered client side based on server side text instructions?

One thing I noticed is that the game does not seem to impose changing the actual ownership of its NFTs.

This is one of the things xaya.io solves with Game Channels and Player Account ID on Polygon. But yes too many games are limited to transfer true on-chain ownership and that's something I know you've wanted to do with in-game items/on-chain items since we began talking. Trustless secure transfer on chain rather than "eBay" auctions like WoW items and risk never getting your items.

That led me to think that maybe if I wanted items that are represented by NFTs to be left behind when a player moves from one planet to another, I might need to have the planet the NFT is left on have ownership of the NFT, at least until the player returns to (attempt to?) reclaim it.

This is brilliant, its a warehouse receipt that the player takes to the other planet and when they return, they can claim ownership or use (of land in your example) when they return because they have the 'token' aka warehouse receipt/title/deed to the property left on the planet they end up returning to.

Consider land for example. If a player buys a tract of land on one planet then moves "personally" to another planet, would it be "reasonable" for the land to go with them?

No- very much so easier to have 'token' in the player's wallet representing their sole ownership of the item/land etc. in form of 'receipt, 'deed, 'title, etc. as commented previously so upon return, land is there and still theirs.

Certainly it could make sense for them to take with them some kind of title deed or even NFT attesting to the fact that before leaving the planet, and thus the land on the planet, behind some kind of authority or government or military force or whatever on that planet had regarded that land as in some sense or other "theirs"; but bringing the actual land with them?

Looks like we're on the same page here. Cool :)

Thus the items the "source code" of the "room" specifies as the contents of the room get spawned over and over and over again, potentially endlessly.

Sounds diluting. If an item must be "rare" to have some sort of value to the player outside the game (on chain/auction/trade) - which is a motivation (admittedly) for a person like me to even play a game to collect said loot - the "rarity"... Sorry, never way much of a gamer but you're stuck with me :)

Which can be a massive influx of items / goods into the "economy".

In 1536 Charles I of Spain and also Charles V Holy Roman Emperor (amongst other titles) authorized the first North American mint to make silver coin to facilitate trade in the new world (Mexico City). He also had the first mass treasure fleet missions to South America and back to Spain flooding the European market with silver - the first ever modern instance of inflation of a commodity metal (and mostly/eventually a coin known as a real, daaler, and thaler throughout Europe). This is the problem of the past and meta-present!

(Economy?! What economy!? MUDs don't have economies, they are "Monty Hall giveaways" of stuff-out-of-nothing!)

Too bad they are only tokenized into HORIZON for the moment. I have nothing to do with my mgold ;) - I'm patient though. Can't wait for the gamemaster to delete all the extra mgold that people didn't want to put on the platform.

Well well well. I have struggled with that in CoffeeMUD, trying to think where the "monsters" that CoffeeMUD spawns are supposed (backstoried; rationalised) to be coming from; maybe I can tie them to Splinterlands NFTs!

Imagine if you will an area-building service for CoffeeMUD that populates an "area" (collection of "rooms"; collection of locations) for you with the monsters depicted / described by the Splinterlands NFTs that you "pay" to that service.

See what I did there?

Yes. Exchange from wallet to some wallet the game receives the "token" representing the monster (the "NFT") and give the player an in-game "monster" to use from the spawn room so they can play the game using the spawned in-game monster.

Rather than do the potentially complicated thing of trying to go look at the buyer's own collection of NFTs whenever the "rooms" are to be populated, I jumped directly to simply accepting the NFTs themselves as "payment" for populating the "rooms" with "corresponding" monsters!

The old switcharoo. "Warehouse receipt" :P