Lightflame
Hmm… I honestly thought he just died of old age… But now, hearing there's an e-book planned regarding his death, I'm thinking that's probably not the case.
It's be a very boring novella, if it were.
I like the idea of him dying from sickness. It'd be cool to see a mercy killing, though I'm not sure they'd put something like that in a kids' series.
I think we can definitely rule out old age, because Ravenpaw's Farewell takes place during Power of Three, apparently, and Dustpelt was still doing fine then.