Nexus 6 and Nexus 9 are already getting official TWRP custom recovery, as new posts show. Team Win Recovery Project version 18.104.22.168 is ready for download and all you have to do is go to TWRP to get it.
Below you can find the changelog for the update itself as stated on the official page:
- Pull in all changes from Android 5.0 lollipop into TWRP
- Add decrypt support for Android 5.0 lollipop encrypted partitions including automatic decrypt when the default_password is in use
- Revert some changes to exFAT that were breaking exFAT support on some devices
- Other minor fixes and updates
In order to install the custom recovery all you have to do it unlock the bootloader and flash the .img recovery file in fastboot.