In a recent interview, Notch mentions some possible up-and-coming changes for 1.8, such as boss fights, clearer boundaries and features for biomes, and more. On Terraria-style inspired bosses, he said that "“...might end up somehow in Minecraft ... I really like that idea.”
Additionally, he mentioned the future possibility of Minecraft resembling Roguelikes, stating that having a goal, however difficult to achieve, is often a driving motivator for players, to give them a purpose in continuing on.
Stay tuned for more information!