According to the buddhism of Nichiren Daishonin, which is a japanese mahayana tradition, the buddha nature is something we all have, at least in latent form. So I agree with the above poster in the sense that the buddha nature, or buddhahood is a quality intrinsic to all living beings - and even non-living things that are also part of this universe - like the emptiness you mention - which being empty, has the potential to manifest life too. It manifests itself when you're in rhythm with your life and environment and are able to bring out the best in you to change your karma and overcome whatever obstacles you were facing. So basically it's when you're being a "hero of your world", and devote your life to the teachings of the lotus sutra. This sutra, or, the middle volume of it, says all prior teachings were "expedient" ways to get people practicing buddhism according to the time, but that actually you don't need lots of lifetimes to become a buddha but can be one now in your present form.
In this way it might relate to concepts of soul in hinduism although I think wikipedia explains it better.
The mystic law of cause and effect, the way we nichiren buddhists understand it, is basically a pervasive universal vibration or rhythm, a sound, a teaching, working at a fundamental level that we can all tune in to, and I believe it is the collective will that the universe has always had: its wish to grow, learn, flourish and be generally positive. So it's a decentralised vision of "soul" - more like the "force" in star wars (without those new films and their midichlorians, ugh!), or cells in a body. Buddhahood, if seen as something we all have, is something we can all tap into if we have the luck to figure this out ourselves somehow, or hear about it, and we can all try and manifest. I also believe buddhism itself is kind of just common sense, and just wisdom and culture, built up over the ages and continents, which has created a common understanding of the world around us, and a philosophy for how to live it. When we die we see it as drops falling back into the ocean. This is why we live forever: Because we are able to live as long as there is life. We are part of all of it and identify with this concept of unity - as if the first and fundamental mistake even the simplest bugs make, is to think of themselves as separate from what's around them.