24 nVersion bits for general purpose use.

Would set aside 24 bits of each block header as spare room for mining hardware, more than BIP 320's 16.

Mining chips try huge numbers of variations of a block header to find a valid one. BIP 320's 16 spare bits proved too few, so some devices have started varying the timestamp instead. This would reserve 24 bits for that job and stop using them to signal upgrades.