Also Psionic Bone Runes could provide living creatures with built in propulsion, by inscribing a psijamming helm, or the propulsion method of a Stronghold Space, or any other motive effect from a psionic item into a creature.
Furthermore, psionic telekinesis works for movement in outer space, as acceleration at near distances, and psionic teleportation works for medium distances, especially if either is crafted as an at will effect, and finally Greater Teleportation works for interstellar travel, so long as you get the visualization right and don’t pop in too close to the star.
Lifeshaping could also work, but it would work even better if you include some science. Perhaps my heresy idea of ancient astronaut aliens being the source of all humanoids on Athas would fit here. That would allow us to incorporate some of the d20 future psionics and mutations as part of lifeshaping effects.
Mutations can get us non-psionic and non-magic telekinesis and teleportation.
Photovoltaic bio-crystals can convert light to electricity which can then be converted to power via energy absorption or a variant version of it. This could also provide sustenance effects as well as absorb radiation and cold damage for energy, while handily protecting one from the rigors of space. Not realistic, but that’s D&D rules for you.
If we allow science, then we can tap d20 Future for converting devices into lifeshaping.
A variation on Cleansing Wars campaigns would be one where all the players are from the same race - dwarves, elves, gnomes, trolls etc fighting in one of the wars between races before Rajaat becomes the Warbringer.
One of the things I liked about the Dead Cities and Trembling Plains works is the idea that the dwarves of Hogalay were in conflict with local gnomish populations over differing interpretations of the Earth faith. Goblins and almost every other race would war, ditto for orcs.
But those wars of the Green Age would be ‘normal’ wars - fought for control of resources or to check a rival’s influence. You can do a war campaign without it being the, frankly, horror story of genocidal wars to the utter death. You can also do it in the knowledge that these petty wars were all the more tragic for the whirlwind of destruction that was coming down the line.
Holy crusades in a world with no Gods but only Elemental Princes is giving me an Avatar the Last Airbender vibe. The Fire Nation Attacks!
Or your idea of different worshippers of the same element having sectarian differences is even better. “Do you worship the Earth Shaker?” “No, I worship the Father of Mountains!” “Die, heretical scum!”
@JesseHeinig has a 2E elemental cleric kit based on element bending. Decently balanced too, by the look of it. I think you can find it if you search him in the Dark Sun group on FB.