I think it's Wylie, because there's a "beloved friend" who's in danger and at the same time, Wylie appears on his first cover. But that's probably because I'm kind of obsessed with Wylie.
Silveny is a good option though, because she's probably going to give birth to the baby alicorn in Flashback. That's a really good reason for her being in danger. If the Neverseen or someone else manage to capture Silveny and kill her, the Lost Cities will have lost their symbol of hope. And they'll also be on the path to destruction, whatever that is.
It just better not be Dex.