Testnet 5.

Would set up Testnet 5, a testing network to replace Testnet 4, with no low-difficulty shortcut for miners.

Testnet 4 let a block be mined at the lowest difficulty if 20 minutes passed without one. This was exploited so much that the latest blocks kept being replaced. Testnet 5 drops that exception, raises the minimum difficulty and applies the BIP 54 rule fixes from the first block.