Blizzard seems to be preparing to introduce full-blown customizations for Balance Druids' Moonkin Form with Patch 10.2 or later. Today's PTR build adds the base of the system to the game, but it isn't fully testable yet.
This build adds the following customization categories, likely all to be used by the Boomkin forms:
Full Transformation likely encapsulates forms that deviate from the base one, like Kul Tiran or Zandalari Troll forms.
With all these customizations, Druid can enjoy a new Glyph, allowing them to look different every time they shapeshift into Moonkin Form.
While the customization isn't 100% working on the PTR, we've prepared a couple of examples of how you can change your Boomkin based on datamining below.