Back when I did some original signs for some fantrolls I was gonna use for an MSPFA that ended up never happening, I sorta just approached it like making a logo or something, or a super hero symbol. I'll take some aspect of their personality, or appearance, and then merge it with elements common to the zodiac sign of their caste.

For instance, I had this character that was a communist theory nerd. She was a teal blood, whose sign has that omega shape that forms the scales in Libra. So I made a similar looking shape, but made it look like a sickle, then added a bar crossing it.

I also recommend looking into alchemical signs for inspiration, as astrology and alchemy have a lot in common. They give a pretty good idea of what you can do with simple shapes, and sometimes you can take aspects you like from different ones and merge them together into something new.