Hm.. I think I understand how the rules work for this. Okay then, here we go. I have a few stories and none have a cover but I'm just going to list two for now.

First one would be Eternal Night. The cover art I'm thinking of would probably be a slightly older looking Twilight protecting a filly Luna from what appears to be a band of unicorns not unlike Sombra on the left side of the image and her lying defeated before an aggressive looking Celestia with what appears to be a stake/spike of light through her heart atop the Castle of the Royal Pony Sisters on the right. Title of course on the image if it can be fixed in such a way.

Second one would be Chessboard Gambits. I'm really open to anything on this one to be honest. I would say do something based on a chessboard but there are about a dozen stories that already have that for their cover art so I'll leave it up to the imagination of the artist. Just keep in mind that ideally it should show Twilight as the victorious piece/pony over pretty much Equestria, Celestia, and House Blueblood at least.