Personally what I'd be interested in in a new expansion would be a space equivelent of Epics. For that purpose, I invented Cosmic Horrors.
According to Spore, Epics go through several stages-
Lone creatures. When you extinct a species, sometimes some of them will be left behind.
Evventually, one or more of these may become rogues.
Occasionally, a Rogue will become an Epic. Very rarely, an Epic will get hit with a mysterious ray from the sun. This begins his cosmic horror transformation.
The Epic will, after a few years, become a Cosmic Epic. Cosmic Epics float on their planet, are a lot creepier than epics (their legs are procedurally taken off as well as their eyes, and all parts are warped to creepier versions), and have about 2.5 times as much health. Their attacks on space ships are several times as strong. A few more years, and it will become an Epic Horror. They still just float on a planet, but they have 10 times as much health as a Cosmic Epic and can one-hit weaker spaceships. They are also even creepier than Cosmic Epics. Finally, the Epic Horror becomes far, far larger, larger than even the Spore planets, and its health increases to ridiculous extent. It will float from solar system to Solar System, eating planets unstoppably. It can one-hit anything, even if you have a sheild activated. Just to show how large they are, you can land on them. However, a quick cutscene immeadiately follows in which you are absorbed into its very skin.