> For the Pubkey field, use a recent Bitcoin mainnet block hash.
Do you mean taking the hash of the block, and using it as x-value for a public key? Then, it could be potentially spendable, if it would be a valid secp256k1 point, and people would send more coins to it, or unspendable, if it would be outside of the curve. For example:
spendable: 0200000000000000000001799c6f72ebe5ebc6b18fd5cdf8bd3697b8d73d01b084 OP_CHECKSIG
unspendable: 02000000000000000000013ac2955a2b6029bc1a86ab4b19e01e8030ceb0eeb2ae OP_CHECKSIG
unspendable: 00000000000000000001799c6f72ebe5ebc6b18fd5cdf8bd3697b8d73d01b084 OP_CHECKSIG
Also, if the size of the stack push would be different than 33 or 65 bytes, then it would be unspendable (for example if it would take 32 bytes, like block hashes, and would be followed by OP_CHECKSIG, to invalidate it, or if it would start with OP_RETURN).
> Difficulty: 0x1d00ffff
I wonder, how quickly it would be listed on exchanges, and traded. Because historically, new testnets were launched, when that happened. But today, new coins are listed faster, than they are replaced. Which would give the creator with any ASIC an incentive, to mine initial 30k or so blocks, keep them for a while, and then sell for BTCs, when the coin will be listed.
Also, I wonder if testnet5 will have any premine. Previous attempt to create testnet5:
https://bitcointalk.org/index.php?topic=5543921> Testnet 5 also does not apply the difficulty exception rule from Testnet 3 or Testnet 4 requires.
It would be nice to simplify the code, and remove that rule completely from sources, but it would probably take a while to deprecate testnet3 and testnet4.