As I started reading this, I was thinking that what might be fun might be some kind of Order mission or other action-y scene for Tonks to show her stuff in.

But then I got to the part about Luna and the thestrals, and that sounds like a really excellent idea to me... interaction between those two is bound to be interesting -- presumably Luna remembers Tonks from the fight at the Ministry in OotP? -- and an encounter with Luna (and the thestrals) might well be an important piece of Tonks dealing with the loss of Sirius.