I think it'd be cool if one could actually make any hair style, and choose specific lengths and decide, hey, I want this patch pinned up here, or maybe I'll braid this and, and have this braid behind her ear. But I realize that it'd be extremely difficult to simulate hair. But maybe have it so that hair will bounce as you run. But probably easiest is to play with one's hair. It'd be cool if a nervous sim will take a lock of hair and twist it around there finger, and the hair would aromatically be attached to the head, it'd be funny to see a male with short hair to suddenly pull out a shoulder length lock of hair.
Maybe a gender confused trait, which would make your sim act abnormally like the other gender, but have nothing to do with who the sim would be romantically attracted to. Or an obsessive trait! That would make your sim more likely to become attached to something, and if it gets sold they'd mourn it. But their obsession wold be randomly chosen from among the things your sim uses the most.Something else cool would be an old school expansion featuring objects from the sims 2 and the original sims, like the Murphy Bed (fold up bed Sims 2 Apartment life), Vending Machines (Sims 2 Apartment Life), Massage tables( Sims Superstar and Sims 2 Bon Voyage), Sky Diving Simulator (Sims Superstar), etc. And Aliens!