Signet is one idea, sidechain is another one. For now, I am more focused on sidechain, because: 1) signet require signing blocks, it means that developers should give coins to the users through faucets, so it is no longer permissionless 2) having too small difficulty means that attacking the network is easy, having too high difficulty means that mining mainnet is more profitable and testing is costly 3) reusing coins from mainnet in testnet means that miners can get two coins at once, which is perfect, also difficulty matches for both coins 4) destroying coins from testnet when mainnet coins are moved means that testnet transactions could be deleted after testing if needed 5) there could be more than one sidechain, for example now we would have taproot sidechain, but later we could get rid of it (just by moving coins on mainnet) and create some other testnet for other features, that means upgrading is easier, no voting, no signalling, just moving coins from one network to another with single mainchain transaction But of course, sidechains needs more coding, there are some existing examples like Truthcoin, but still, if we need to start it right now, signet is of course easier to set up. Testnet is even easier and no software changes are needed to start mining on testnet right now.