ZFS Install, add hot spare option
Since the 2.4 beta installer has the option to setup ZFS, and a lot of parameters, I would like to see the installer have the ability to handle adding a hot spare to the to-be-created zpool. Doing this after the fact is less convenient than doing it during install. This is primarily due to the partitioning done during the install for the boot/swap/zfs partitions. This partitioning makes "zfs replace" pointing to a specific disk a bad idea, as you need it to point to a specific partition.
I see having it in the installer means the hot spare will already have the appropriate partitioning done to it, and having the pool already list it as a spare. I think this convenience would pay off.