[gertrunde]

The distro of your choice from the small pool that you are given to choose from.

It's not a completely unrestricted choice.

That pool includes Ubuntu, Debian, SUSE, Fedora and a few others if I remember correctly, although it may have expanded since.

I'm not sure how straightforward rolling your own is.

[Atomskun]

You have no idea what you are talking about. You can install any kind of distribution by copying over their rootfs. For example, I am running Void Linux on WSL2.

[int_19h]

The pool in question is open - anybody (not just Microsoft) can package and publish a distro there.

And for one-off use, there's wsl --import: https://docs.microsoft.com/en-us/windows/wsl/use-custom-dist...