A thief in Thunderclan

So am I! Brightheart's POV along with amazing art will be so awesome to see.

I hope the thief is Ashfur

Doubt it, it looks like this book will take place after he dies

There has been no explanation as to when exactly the book will take place except for after the dog attack and that Brightheart looked to be expecting kits in one shown panel. But the possibilities for it to either play out in the old or already new territories are still wide open as well as timeskips or even flashbacks. While the mention "moons of battle and betrayal" can very well refer to Tigerstar's reign and the BloodClan battle in which Darkstripe died, there is yet to be an actual confirmation.

Perhaps in the end, neither Darkstripe nor Ashfur (in reference to you liking Spinestar's comment about him) are revealed to be the culprits and it's someone completely different. Just saying. Because I personally don't really see why Ashfur would steal prey from his own Clanmates. Not even if the case is that this GN happens during the PoT era where he went completely off the deep end. There's simply too little info out as of now and still a lot of room to theorize further instead imo.