Krypton opens an entire segment of the DC Universe that’s only been scratched by the other live-action shows (primarily through Supergirl). The Last Daughter of Krypton lives on Earth 38 where alien intervention is so common they have an entire organization, D.E.O., dedicated to Earth’s defense (most of our other heroes live on the alien-free Earth-One). Thanks to the season-ending cliffhanger of Krypton (which doesn’t take place in the Arrowverse) we can now look to the stars and beyond. In case you stopped watching after the first couple of episodes, here’s what you missed: General Zod, who traveled back in time under the ploy of stopping Brainiac from bottling Kandor (who the viewers had been led to believe had traveled back in time), is successful in his mission. He manages to alter the timeline so Kryptonians can conquer the stars thereby creating the possibility of more space-faring species and characters. It also, apparently, means a future without Superman.
One tiny catch, Krypton takes place 200 years before baby Superman was sent to Earth, so it does limit who could show up based on lifespan. Although Adam Strange could start playing with time travel to bring heroes back who could help restore the timeline and defeat Zod.
Assuming we’re solely dependent on what exists at the time of Krypton here are some of the alien species or characters who could make an appearance in future seasons of Krypton.