Genetic Modification

Story-wise, genetic modification is usually centred on humans, probably the most subtle and creepy way of messing with mankind. Playing with the fear of the unknown, genetically altered humans are technically a new species, that may not only surpass but actually replace unaltered humans. The level of modification will vary from subtle changes in "look" (e.g. super-human beauty), to mental enhancements (PSI powers), to more or less atrocious adaptations to extreme environments (breath under water, radiation resistance). Note that changes in Aliens would be difficult to pinpoint, after all they are alien.