Do nothing except implement the kernel correctly.
There's plenty enough there and be aware that it's not a Linux kernel these days unless cgroups, netlink etc actually work properly. Use ordinary filesystem files as block devices.
Paul Buonopane commented
We still need some way to actually access the kernel. Currently our only way of interfacing with it after installation is bash.exe. As far as I'm aware, there isn't any documentation about how Windows programs can access Lxss, or even whether such APIs will be available.