More types of marriage, based on other cultures.
New types of wedding clothes: chinese, indian, japanese, classic dresses, gothic dresses, etc.
The ceremonies would be different as well as the types of activity for the wedding party. It would also depend on the world you would choose to marry.
You could also be "baptized" by another culture, depending on your cultural skill. ( If you have been baptized by Chinese culture, you can marry in Chinese style with anyone, anywhere.)
PS: I really wanted a Sim to have his own ethnicity, depending on which world she was born or started her game.
But if this is not possible, the option to convert to another culture would also be useful.