Innistrad, the newest MTG set is looking to be quite interesting. Not only does it support a classic "monster movie" theme filled with Vampires and Werewolves, but it also introduces a new mechanic into the game. This new mechanic, transform, allows for a card to transform into something else once a specific condition is met.
But how does it transform? Easy, the cards now have designs on both sides so transforming a card is as simple as flipping it to the opposite side.
Creatures that transform normally tend to transform back when other conditions are met as well. However, this makes for quite the interesting mechanic because you can end up with a powerful creature when you least expect it. Not just that, but transformations aren't just limited to creatures. Planeswalkers aren't safe from it either.
And all transforming cards must be played in their basic forms. Transformed cards don't have any mana cost, and it's understandable.
This new mechanic is quite interesting and I want to see how it shapes the game in the future.