One idea that shows up in a lot of speculative fiction is the idea of Humanity transcending a single biology, through a variety of different technologies. In this concept, the human form is mutable and individuals can shape their body to conform to an environment, or even just a personal preference. Often this is accomplished through nanotech, consciousness transference, genetic engineering, or a combination of any number of those elements. And with the release of Alien Archive 4, you can set your Starfinder game in just such a future.
The introduction of species grafts allows a greater breadth of customization for players, letting them add the biological advantages of a different species to their own character, for a price. For a this setting, I propose taking it a step further. Base races allowed are Human and Android, but at character creation every player has a budget of 3200cr they can spend on any combination of Species Grafts or Augmentations (excluding Personal Upgrades) and they are not limited by level for these purchases. If they want to trade in one of their existing traits (Bonus Feat and Skilled for human, Exceptional Vision and Upgrade Slot for android) they may do so at character creation for an additional 1750cr. Any credits of this budget not spent at character creation are lost.
Due to the commonality of body customization, players have broad leeway in how their character looks and how their biology operates, but this does not have a mechanical in game effect.
Whether you want magic to exist in this setting, reflavor that to just being psychic powers, or removed entirely is up to you. Similarly, undead and the metaphysical aspects of deities and planar travel may or may not have a place in this modified setting. Make sure you have a firm idea what you and your players are looking for from the setting, and that everyone has an understanding what is and is not allowed.
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Until next time Starfinders!
Artist and Writer