It's been said before - it has been used synonymously in various Mahayana schools with dependent origination, emptiness, unborn, unconditioned, the potentiality for awakening, the luminous mind free from defilements, freedom from delusion, unbinding and the liberated mind being no different to the mind of the Buddha. It is not a "thingie" in most treatments, it is simply a pointer to awakening. It is what happens when the defilements, obscurations and ignorance are removed.
I don't think it is reasonable to declare every phrase and device used by a teacher heretical if it is not found verbatum in the Canon. What matters is the import of the teaching, what it is pointing towards. Just like intention with sila, so it is with the teachings - if they point towards liberation from delusion, they are Dhamma.
Of course not being liberated, we do have a tendency to cling to words - make a fetish of the raft rather than use it for its intended purpose. I don't recall my teacher use Buddha nature in her teachings, but for other teachers and students it may be appropriate.