Consider this: Theravada posits Nirvana as a _remote_ goal, while Mahayana posits Buddha-Nature as _self-existing_ state of affairs. This is the key.

The reason Gautama Buddha declared Nirvana, is because he was modeling the Path after his own quest. Because he achieved his Enlightenment after having gone through numerous trainings and realizing Three Marks of Existence, Twelve Nidanas and Four Noble Truths, he saw that all students must go through the same experiences in order to achieve the same Liberation. Indeed, for someone without basic discipline, critical thinking skills, and capacity for self-reflection, attaining Buddha-Eye is outright impossible.

By the time of Mahayana though, because Buddhism became very popular and the essence of Teaching was somewhat lost in the noise of speculative philosophy and esoteric speculation, many new students would become obsessed with the _idea_ of Nirvana, and instead of training the skilfull qualities of mind required for Enlightenment, would engage in fruitless search of the Transcendental Realization. Out of compassion for future seekers, Bodhisattvas established the notion of Buddha Nature, openly declaring that Enlightenment is not something remote you have to get to, but rather one's innate nature one has to recover.

So Buddha-Nature is basically Nirvana or Enlightenment, except now you know you already have it, and only need to open your eyes, instead of running around the world seeking it. That said, you still have to work on dropping all attachments, deconstructing the ego, and becoming the master of your mind/emotions before you can take legal ownership of your rightful possession :)