Add support for logical volume management & abstraction to the Kernel & bootloader
While Windows has a lot of advantages over Linux in a lot of areas, one area where Windows is still extremely weak is in volume-managment and filesystem management. While support for more kinds of filesystems in Windows would be nice, one thing Windows should add is proper logical volume management to Windows.
In Linux, LVM support can be added to the Kernel as well as the bootloader, so you can do things like resize and extend partitions without worrying about silly things like partition-ordering and gaps, or the difficulty of extending a partition leftwards rather than rightwards on the disk (doing so now in Windows will leave the drive unbootable).
Moreover, moving a whole Physical volume in LVM (each Physical volume corresponds to a physical partition on a disk to another disk is as easy as setting an LVM flag on a partition on another drive, adding the physical volume to the Volume Group, and removing the original physical volume from the group.
Probably to get the most out of this, it would be really nice if NTFS could be upgraded to be a bit more flexible with regards to resizing up and down.