Imagine your faves in a world where birth charts legally rule your life, whether they're accurate or not. Literally star-crossed lovers of conflicting signs hide their love and fight to be together. Pick a birthday for your faves who don't have a canon one and get to it.
I've already got Papyrus pegged as July 30th for his birthday (international day of friendship and a Leo)
i don't think i have a headcanon for norm the genie's birthday yet, but the birthday of norm macdonald, who he was based on, is october 17, which would make him a libra.
A few of my fave ships in Homestuck clash with standard astrological theory, so either humanstuck or trolls-on-Earth would work there. Maybe it could also work with characters being forced into careers they don't want because it's auspicious for their signs? (Always wanted to be a lumberjack.)
Potentially gets doubly complicated upon introduction of the Chinese zodiac. I find that fascinating too, and the combination of both comes up with some interesting stuff.