It makes me wonder how much more is out there I've never seen in GUI wallets.
For anchors, introduced in the latest version, you have only this single address. But: there are many possible scripts, where no keys are required, for example:
https://mempool.space/testnet4/address/tb1qft5p2uhsdcdc3l2ua4ap5qqfg4pjaqlp250x7us7a8qqhrxrxfsqaqh7jwWhy would ASIC miners do that?
Because in this way, you peg coins into other networks, based on Proof of Work. This kind of locking script is just "lock to a sidechain output", which would be moved, when sidechain users would produce small enough signature, to move those coins somewhere else.
Or better: why are they mining testnet coins in the first place?
Because it is profitable. Normally, everyone would mine just Bitcoin, and nothing else, because there is no reason to mine anything else in the first place. But: because the whole game is just a "winner takes all" game, and because making altcoins, mining them, and then selling, is profitable, then that's what people do. And now, testnets are just altcoins, signed by Bitcoin Core developers, so unless the whole model behind testnet will be changed, they will be pumped and dumped endlessly, because it is profitable.
In the ideal case, you wouldn't have other mining algorithm than double SHA-256 in altcoins. And all of them would use Merged Mining, so reorging any altcoin would be as hard, as doing 51% attack on Bitcoin. And all chains would calculate "the global difficulty", to make sure, that their chain cannot be attacked, if you don't have 51% mining power in the whole crypto space.
But: instead of that ideal model, you have just completely separate chains, and you can get more BTCs by mining and selling altcoins, than by mining BTC directly. Sad, but true. And that's why people are mining testnet3, testnet4, or any other altcoin: to sell it for BTCs. And of course, some miners can have some alts in their wallet, but then, they keep them only, if they can increase their profits (for example by having bigger control over a given altcoin).
How cool would it be if real Bitcoin has this feature?
1. You wouldn't need any centralized mining pools, if it would be possible to directly mine some weak block, and get a fraction of the coinbase reward, for doing that.
2. You wouldn't need altcoins, created only because of mining. You would just mine Bitcoin, and lock your coins in a different way, to peg them into any subnetwork you want.
3. You wouldn't need any trading, to switch from feature A into feature B. You would just move your coin directly from address A into address B.
I know, I know, it won't work and will only create dust
1. It could work, but nobody put enough effort, to make it real. There are only some local test networks here and there, but nothing, that you can practically use.
2. There is no need to create any dust. If you would have just off-chain transactions, paying you all, what you mined, then you could accumulate your rewards, and withdraw all of that in a single transaction, when it will be worth it. Nobody needs to withdraw a single satoshi immediately.
3. If you make some LN transaction, then it costs you some small fees. If you are a miner, you can be rewarded just in free (or discounted) transactions, inside LN.
There's some charm to seeing coins appear out of thin air.
Well, for better or worse, Garlo Nicon's code still works, and is compatible with many networks (yes, it works on testnet3, too). You can build as long local chain, as you want. Of course, all of that will disappear, but you can build such chains on top of any networks, including mainnet (if you enable minimal difficulty rule, which is just a single boolean in the code). By the way: doing it on top of mainnet has a lot of sense, because then, you have a lot of real data, to test transaction batching, checking if some non-standard transaction is valid or not, measuring your real hashrate, and so on.
In general, if you call "getblocktemplate", then you can get everything you need, to make a valid block. If you would pass all of that into some regtest client, which accepts blocks with any difficulty, then you will get those "coins appear out of thin air", you will have the chance to explore, how transactions are picked, how they are ordered, how they are selected, how long it takes, before your newly-started node will fetch enough transactions, to fill a block, and so on.