well brahman is more of term used in hinduism, but this concept of the "returning to the whole" is also prominent in buddhism, at least from my understanding of it.
First of all, Id like to point out that Brahmin is a caste and the word you are thinking of is BrahmAn, the infinite all eprvading reality, the self of all beings. An elightened person is one who is already absorbed int Brahman, they are independant of the body. The body is cast off like an old set of clothes. It is nopt that the enlightened need to wait for death to merge with brahman, they are already merged with brahman, even when using the body. Also you can never be a PART of brahman, since it is akhanda, indivisible. You become one with Brahman, there is n separation, no duality.
I rather have an honest I dont know than a elaborated story, speculation of what is going to happen, it doesnt matter what happens. We have today to worry about.