The Kraken Wakes by John Wyndham Man vs. aliens in space and across the galaxy. Alternate Worlds Doorways between parallel universes, sometimes similar to ours, sometimes fantastic. The Number of the Beast by Robert A. Heinlein Piers Anthony's Mode books Piers Anthony's Apprentice books The Forgotten Door by Alexander Key Quantum effects cause uncountable branching alternative worlds. The creation of robots for special purposes. Computers or robots long to be human.
Computers provide virtual realities for human adventures.
Long-lived robots or computers are the benign caretakers of humanity. Asimov's later Foundation books. The Gladiator by Philip Wylie Psi powers. Only Begotten Daughter by James Morrow Pegasus in Flight by Anne McCaffrey The Practice Effect by David Brin Clones. Clone by Richard Cowper Other species gain enhancements or mutations, sometimes with bio-engineering.