What cards from M15 are going to have a big impact on Standard
Fact or Fiction became such a powerhouse that it became an anagram for Vintage - "EOTFOFYL" which stood for "End of Turn, Fact or Fiction, You Lose." It led to dominant new decks in Vintage (such as a mono-Blue build called "BBS" for Blue BS which led to Morphing) and was ultimately restricted
Similar to Artemis, Menace of Time, Xande Dark Mage is far from good, but there may be a specific niche that is interested in this card
for instance, the Bello, Bard of the Brambles deck takes its raccoon leader and spins out an entire "one person's trash is another person's treasure" story through its card selection and archetype
Among all the memorable evocations of the genre, arguably nothing felt more flavorful than seeing the townsfolk of Innistrad turn evil under a full moon