Round roof segments - Im not sure if the round roof segments are something that could be done but id love to see them! The only way i can think that they could be done is if they work like the mansion and garden mansard roof pieces.
Mansard roofs - Mansard roofs would be cool to!! At the moment you cant make them as curving the roof with the ball that comes up when the roof is highlighted lowers the height of the roof to low down.
Hexagonal roofs - Hexagonal roofs also would be nice as i haven't really used the hexagonal foundations in game as there are no roofs to put over them that look right.