Usually I do not write about fantasy, but today I would make an exception for Bangsian fantasy. Bangsian fantasy is a subgenre of fantasy which is set in the after-life. According to Wikipedia it usually involves famous historical or literary figures. It is named after author John Kendrick Bangs who wrote such fantasy.
This type of fantasy might be uncomfortable for secular writers as the concept of the after-life has strong religious connotations. There have been, however, atheist philosophers who have presented arguments in favour of life after death. In this I would discuss two of them: David Staume and John McTaggart. Though I will discuss their arguments, I will not argue they are actually right.
David Staume is the author of a book titled The Atheist Afterlife. Staume argues that life after that does not require the existence of god, but only the laws of physics. His argument, as I have understood from what I have read through Google books, is as follows. First he argues for a kind of mind-body dualism. In his version of dualism consciousness is similar to matter and energy, and if so consciousness is similarly subject to the law of conservation of matter-energy. Hence consciousness cannot disappear and therefore it has to last forever.
According to Staume the experience of consciousness is similar to dreaming. It is this what makes his hypothesis interesting for potential writers of Bangsian fantasy.
A few critical notes could be made. If it is true that consciousness is subject to a law of conservation, then consciousness does not have not only no end, but logically also no beginning other than the big bang. It raises the question where conscious does come from and how it interacts with our material bodies.
The second issue I would address is that of solipsism. As I have understood Staume, and in particular his argument that his afterlife is a kind of dream, one should wonder whether a disembodied consciousness could only affect itself, or that it could interact with other consciousnesses. I see no reason why a consciousness could only interact with a body during life and not after death. Since I do not have read Staume’s entire book, I am not aware whether he addresses these issues.
John McTaggart is an entirely different thing. McTaggart, a convinced atheist, was an idealist and his believe in immortality is related to his argument for the unreality of time. Idealism is the metaphysical position that mind is the primary substance, as opposed to materialism which holds that mind/consciousness emerges from matter.
In his view multiple minds, perhaps an indefinite number, do exist. Since McTaggart denies that time has an objective existence, these minds have consequently neither beginning nor end and hence those minds are factually immortal. Though it is not clear, at least to me, what his type of immortality would look like, probably he believed in some kind of reincarnation, it does provide some grasp for the writer of Bangsian fantasy.