I just remembered . . . since you like the plot of Marmalade Boy (stepbrother/sister thing), Akuma de Sourou (aka The Devil Does Exist) might be to your taste. For something similar to the angsty intensity of Mars (but totally different storyline), you should go for Please Save My Earth. It's not exactly my favorite but it has one of the most haunting scenes and is definitely one of the most memorable shoujo series I've come across.
But once you conquer the world, then comes the hard part.
What kind of world would you create?