[@Zran]
I recommend running a theme with your spirits, e.g. based off of a certain group or mythology. The 12 Zodiac Keys are based off of the 12 zodiac signs, Gabriel is off of the Chinese zodiacs and Michael Angels and Demons.
However Celestial Magic is tied with the constellations as well, so probably should look those up first and find some constellations. That was the beauty of Demon Keys, same concept however instead of constellations they're simply demons instead and I went from there.